České vysoké učení v Praze Fakulta elektrotechnická
Bakalářská práce
Simulation model of a Frequency Converter
Mikuláš Jandák
Vedoucí práce: Doc. Ing. Petr Horáček, CSc.
Studijní program: Elektrotechnika a informatika, strukturovaný, bakalářský
Obor: Kybernetika a měření
Srpen 2007
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Abstract This bachelor thesis is intended to implement a simplified simulink model of the frequency converter PowerFlex7000 according to the given specification and to test this model in simple regulations with induction (asynchronous) machines of various power. The model should be tested in both speed and torque regulations.
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Acknowledgment I would like to thank Doc. Ing. Petr Horáček, CSc for supervising my thesis. Also, I am very grateful to my whole family and especially my parents for their patience and support throughout not only the period of my studies at CTU but also my life. Last but not least, I thank my cocker spaniel Daf for his mental support.
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Table of contents: 1. Introduction ............................................................................................................................11
1.1. Historical overview of electric motors ...........................................................................11 1.2. Development of control techniques of induction motors ...............................................11 1.3. Overview of control techniques of induction motors.....................................................13 1.4. Structure of this bachelor thesis .....................................................................................14 1.5. Aims of this bachelor thesis ...........................................................................................15 1.6. Symbols..........................................................................................................................16
2. Functional description of the drive.........................................................................................17 2.1. General description ........................................................................................................17 2.2. Speed control block........................................................................................................18 2.3. Flux control block ..........................................................................................................20 2.4. Current control block .....................................................................................................21 2.5. Line converter feedback block .......................................................................................22 2.6. Machine converter feedback block ................................................................................23 2.7. Motor model block .........................................................................................................24
3. Vector control.........................................................................................................................25 3.1. Transformations used in vector control..........................................................................25
3.1.1. Clark transformation ..............................................................................................26 3.2.1 Park transformation ....................................................................................................28 3.2.2 Mathematical model of induction machines ..............................................................29 3.2.3 Mathematical concept of Vector control and relation to the control of DC machines 35 3.2.4 Vector control in the nutshell .....................................................................................37
4. Implementation of PowerFlex7000 in Simulink ....................................................................39 4.1. Speed Control block .......................................................................................................40 4.2. Flux Control block .........................................................................................................43 4.3. Current Control block.....................................................................................................46 4.4. Motor model ...................................................................................................................50 4.5. Line side converter gating and feedback........................................................................52 4.6. Motor side converter gating and feedback .....................................................................53 4.7. Differences between the model and PowerFlex7000.....................................................55
4.7.1. Speed control block................................................................................................55 4.7.2. Flux control block ..................................................................................................56 4.7.3. Current control block .............................................................................................56 4.7.4. Motor model block .................................................................................................56
5. Conclusion..............................................................................................................................57 5.1. Accomplishments ...........................................................................................................57 5.2. Unsolved problems.........................................................................................................57
6. Literature ................................................................................................................................58
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7. Appendixes.............................................................................................................................60
7.1. Induction machine 3HP: speed regulation below the base speed, both directions, pump type load, Vector control ............................................................................................................61
7.1.1. Parameters ..............................................................................................................61 7.1.2. Results ....................................................................................................................64 7.1.3. Vector control.........................................................................................................68
7.2. Demonstration of the torque ripple and the phase shift .................................................69 7.2.1. Torque ripple ..........................................................................................................69 7.2.2. Phase shift ..............................................................................................................72
7.3. Induction machine 3HP: speed regulation above the base speed, one direction, pump type load, comparison between the flux-constant region and the power-constant region .........74
7.3.1. Parameters ..............................................................................................................74 7.3.2. Results ....................................................................................................................77 7.3.3. Comparison between the flux-constant region and the power-constant region .....80
7.4. Induction machine 3HP, torque regulation ....................................................................81 7.4.1. Parameters ..............................................................................................................81 7.4.2. Results ....................................................................................................................83
7.5. Induction machine 200HP: speed regulation below the base speed, both directions, pump type load, reaction to disturbances ...................................................................................85
7.5.1. Parameters ..............................................................................................................85 7.5.2. Results ....................................................................................................................88 7.5.3. Reaction to disturbances.........................................................................................90
7.6. Induction machine 200HP: speed regulation above the base speed, one direction, pump type load .....................................................................................................................................91
7.6.1. Parameters ..............................................................................................................91 7.6.2. Results ....................................................................................................................94
7.7. Induction machine 200HP: heavy duty start-up, traction type load, comparison of the start-up of induction machines with scalar frequency converters, vector frequency converters and the start-up of DC motors ....................................................................................................97
7.7.1. Parameters ..............................................................................................................98 7.7.2. Results ..................................................................................................................100 7.7.3. Comparison of the start-up of induction machines with scalar frequency converters, vector frequency converters and the start-up of DC motors..............................102
7.8. Induction machine 200HP: torque regulation ..............................................................102 7.8.1. Parameters ............................................................................................................103 7.8.2. Results ..................................................................................................................105
7.9. Induction machine 200HP: torque regulation, uncertainty in all parameters...............107 7.9.1. Results ..................................................................................................................107
7.10. Induction machine 200HP: torque regulation, uncertainty in one parameter...........108 7.10.1. Results ..................................................................................................................108
7.11. Robustness of Vector control ...................................................................................110 7.12. Comparison between 3HP, 200HP induction machine and PowerFlex7000 ...........111
7.12.1. Comparison between 3HP and 200HP induction machine ..................................111 7.12.2. Comparison between the model and PowerFlex7000 ..........................................112
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INTRODUCTION
1. Introduction
1.1. Historical overview of electric motors
The history of electrical motors goes back as far as 1820, when Hans Christian Oersted
discovered the magnetic effect of an electric current. One year later, Michael Faraday discovered
the electromagnetic rotation and built the first primitive DC motor. Faraday went on to discover
electromagnetic induction in 1831, but it was not until 1883 that Tesla invented the induction
(asynchronous) motor.
Currently, the main types of electric motors are still the same, DC, induction
(asynchronous) and synchronous, all based on Oersted, Faraday and Tesla's theories developed
and discovered more than a hundred years ago.
1.2. Development of control techniques of induction motors
Since its invention the induction motor has become the most widespread electrical motor
in use today. This is due to the induction motors advantages over the rest of motors. The main
advantage is that induction motors do not require an electrical connection between stationary and
rotating parts of the motor. Therefore, they do not need any mechanical commutator (brushes),
leading to the fact that they are maintenance free motors. Induction motors also have low weight
and inertia, high efficiency and a high overload capability. Therefore, they are cheaper and more
robust, and they do not tend to any failure at high speeds. Furthermore, the motor can work in
explosive environments because no sparks are produced.
Taking into account all the advantages outlined above, induction motors must be
considered the perfect electrical to mechanical energy converter. However, mechanical energy is
more than often required at variable speeds, where the speed control system is not a trivial matter.
The only effective way of producing an infinitely variable induction motor speed drive is to
supply the induction motor with three phase voltages of variable frequency and variable
amplitude. A variable frequency is required because the rotor speed depends on the speed of the
rotating magnetic field provided by the stator. A variable voltage is required because the motor
impedance reduces at low frequencies and consequently the current has to be limited by means of
reducing the supply voltages. Before the days of power electronics, a limited speed control of
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INTRODUCTION
induction motor was achieved by switching the three-stator windings from delta connection to
star connection, allowing the voltage at the motor windings to be reduced. Induction motors are
also available with more than three stator windings to allow a change of the number of pole pairs.
However, a motor with several windings is more expensive because more than three connections
to the motor are needed and only certain discrete speeds are available. Another alternative
method of speed control can be realized by means of a wound rotor induction motor, where the
rotor winding ends are brought out to slip rings. However, this method obviously removes most
of the advantages of induction motors and it also introduces additional losses. By connecting
resistors or reactances in series with the stator windings of the induction motors, poor
performance is achieved. At that time the above described methods were the only ones available
to control the speed of induction motors, whereas infinitely variable speed drives with good
performances for DC motors already existed. These drives not only permitted the operation in
four quadrants but also covered a wide power range. Moreover, they had a good efficiency, and
with a suitable control even a good dynamic response. However, its main drawback was the
compulsory requirement of brushes.
With the enormous advances made in semiconductor technology during the last 20 years,
the required conditions for developing a proper induction motor drive are present. These
conditions can be divided mainly in two groups:
1) The decreasing cost and improved performance in power electronic
switching devices.
2) The possibility of implementing complex algorithms in the new
microprocessors.
However, one precondition had to be made, which was the development of suitable
methods to control the speed of induction motors, because in contrast to its mechanical simplicity
their complexity regarding their mathematical structure (multivariable and non-linear) is not a
trivial matter.
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INTRODUCTION
1.3. Overview of control techniques of induction motors
Historically, several general controllers have been developed:
1) Scalar controllers:
Despite the fact that "Voltage-Frequency" (V/f) is the simplest
controller, it is the most widespread, being in the majority of the industrial
applications. It is known as a scalar control and acts by imposing a constant
relation between voltage and frequency. The structure is very simple and it
is normally used without speed feedback. However, this controller doesn’t
achieve a good accuracy in both speed and torque responses, mainly due to
the fact that the stator flux and the torque are not directly controlled. Even
though, as long as the parameters are identified, the accuracy in the speed
can be 2% (except in a very low speed), and the dynamic response can be
approximately around 50ms.
2) Vector Controllers:
In these types of controllers, there are control loops for controlling
both the torque and the flux. The most widespread controllers of this type
are the ones that use vector transform such as Park transform. Its accuracy
can reach values such as 0.5% regarding the speed and 2% regarding the
torque, even when at standstill. The main disadvantages are the huge
computational capability required and the compulsory good identification
of the motor parameters.
3) Field Acceleration method:
This method is based on maintaining the amplitude and the phase of
the stator current constant, whilst avoiding electromagnetic transients.
Therefore, the equations used can be simplified saving the vector
transformation, which occurs in vector controllers. This technique has
achieved some computational reduction, thus overcoming the main
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INTRODUCTION
problem with vector controllers and allowing this method to become an
important alternative to vector controllers. Nonetheless, identifying
parameters is still an issue and as the previous control technique Field
Acceleration method also hugely depends on knowing parameters of the
stator and rotor windings.
4) Direct Torque Control:
DTC has emerged over the last decade to become one possible
alternative to the well-known Vector Control of Induction Machines. Its
main characteristic is the good performance, obtaining results as good as
the classical vector control but with several advantages based on its simpler
structure and control diagram. DTC is said to be one of the future ways of
controlling the induction machine in four quadrants. In DTC it is possible
to control directly the stator flux and the torque by selecting the appropriate
inverter state. This method still requires further research in order to
improve the motor’s performance, as well as achieve a better behavior
regarding environmental compatibility (Electro Magnetic Interference and
energy), that is desired nowadays for all industrial applications.
1.4. Structure of this bachelor thesis
The work presented in this thesis is organized in five main parts. These four parts are
structured as follows:
Part 1 is entitled “Introduction." and it gives overview of main control techniques used
both in the past and nowadays.
Part 2 is entitled “Functional description of the drive." and it sums up the most significant
parts of the frequency converter PowerFlex7000.
Part 3 is entitled “Vector control.". It is devoted to introduce control approach
implemented in PowerFlex7000 and shows similarities and differences not only between scalar
and vector control but also between controlling AC and DC motors. It also gives short overview
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INTRODUCTION
of mathematical model of the induction machine and transformations which support Vector
control.
Part 3 is entitled “Implementation in Simulink". It describes the structure and main blocks
of the model of Powerflex7000 in Simulink and it points out the differences between the
frequency converter and its model.
Part 4 is Appendix which shows simulation results. This part also includes comments and
with help of figures describes problems with which I had to deal and tries to come up with
possible solutions. This part also tries to show advantages of Vector control.
Last part, which is Conclusion, summarizes things which have been accomplished and
things which have not been solved.
1.5. Aims of this bachelor thesis
The main aim is to design Simulink model of PowerFlex7000 with emphasis on external
behavior. Nonetheless, this work should also emphasize the properties of vector control and show
advantages over scalar (V/Hz) control. This work is also supposed to be used in the following
application regarding digging wheel excavator SchRs 1320/4x30 which includes two frequency
converters Powerflex7000 and two motors Siemens ARNRY-6 (1000kW, 6000V, 118A, and
1000rpm).
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INTRODUCTION
1.6. Symbols
In this thesis I will use following notation:
x complex number
sL stator inductance
slL stator leakage inductance
rL rotor inductance
rlL rotor leakage inductance
mL magnetizing (mutual) inductance
sψ stator flux
rψ rotor flux
mψ magnetizing flux
si stator current
ri rotor current
mi magnetizing current
mω mechanical angular velocity
cba iii ,, stator current – phase a,b,c
cba uuu ,, stator voltage – phase a,b,c
scsbsa ψψψ ,, stator flux – phase a,b,c
rcrbra ψψψ ,, rotor flux – phase a,b,c
s slip
Te electromagnetic torque
Tl load torque
d,q indexes referring to d,q coordinate system
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FUNCTIONAL DESCRIPTION OF THE DRIVE
2. Functional description of the drive
2.1. General description
The PowerFlex7000, which is shown in Figure 1, is an adjustable speed ac drive in which
motor speed control is achieved through control of the motor torque. The motor speed is
calculated or measured and the torque is adjusted as required to make the speed equal to the
speed command. The parameters of the motor and the load determine the stator frequency and the
drive synchronizes itself to the motor. The methods of control implemented in PowerFlex7000
are known as sensorless direct rotor flux oriented vector control and full vector control. The term
rotor flux vector control indicates that the position of the stator current vector is controlled
relatively to the motor flux vector. Direct vector control means that the motor flux is measured, in
contrast to the indirect vector control in which the motor flux is predicted. This method of control
is used without tachometer feedback for applications requiring continuous operation above 6
Hertz and less than 100% breakaway torque. Full vector control can also be achieved with
tachometer feedback which enables motor to operate continually down to 0.2 Hertz with up to
150% breakaway torque. In both control methods, the stator current (Is) is split into flux
producing component (Isd) and an orthogonal torque producing component (Isq) which are
controlled independently. The aim of vector control is to allow a complex ac motor to be
controlled as if it was a simple dc motor with independent, decoupled field and armature currents.
This allows the motor torque to be changed quickly without affecting the flux. Vector control
offers superior performance over volts/hertz type drives. The speed bandwidth range is 1-15
radians per second, while the torque bandwidth range is 20-100 radians per second. The
PowerFlex7000 drive can be used with either induction (asynchronous) or synchronous motors.
PowerFlex7000 consists of the following parts:
1) Speed control block
2) Flux control block
3) Current control block
4) Line converter feedback block
5) Machine converter feedback block
6) Motor model
7) Line converter
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FUNCTIONAL DESCRIPTION OF THE DRIVE
8) Machine converter
Figure 1 – High-level scheme of the PowerFlex7000 control system
2.2. Speed control block
The function of the speed control block, which is shown in Figure 2, is to determine the
torque producing component (Isq) of the stator current (Is). The inputs to the block are the Speed
Reference from the speed ramp and the Stator Frequency and Slip Frequency from the motor
model. If the drive is installed with an optional tachometer, then the motor speed is determined
by counting the tachometer pulses. In Sensorless operation, the Slip Frequency is subtracted from
the Stator Frequency and filtered to determine the Speed Feedback. In Pulse Tachometer mode,
the speed is determined directly by using Tachometer Feedback. The Speed Feedback is
subtracted from the Speed Reference to determine the Speed Error which is processed by the
speed PI regulator. The gains of the regulator are based on the Total Inertia of the system and the
desired Speed regulation Bandwidth. The output of the speed regulator is the Torque Reference
whose rate of change is limited by Torque Rate Limit. The calculated Torque Reference is
divided by the Flux Reference to determine the torque component of the stator current Isq
Command. To calculate the torque producing current supplied by the inverter Iy Command, the
current supplied by the motor filter capacitor in torque production (orthogonal to motor flux) is
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FUNCTIONAL DESCRIPTION OF THE DRIVE
calculated and subtracted from Isq Command. In Sensorless mode, the drive uses Torque
Command 0 and Torque Command 1 for an open loop start-up. At frequencies greater than 3Hz,
the drive closes the speed loop and disables the open loop start mode. In Pulse Tachometer mode,
the drive is always in closed loop. The maximum torque a drive can deliver in motoring mode is
determined by Torque Limit Motoring. In regenerative mode the torque is limited to Torque Limit
Braking. Depending on the application, the drive can be configured in different torque control
modes by setting the parameter Torque Control Mode. Table 1 shows different torque modes of
the operation.
Torque Control Mode Function
0 Zero toque 1 Speed regulator 2 External torque command 3 Speed torque positive 4 Speed torque negative 5 Speed sum
Table 1 – Torque modes of the operation
Figure 2 – Speed control block
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FUNCTIONAL DESCRIPTION OF THE DRIVE
2.3. Flux control block
The function of the flux control block, which is shown in Figure 3, is to determine the
magnetizing component (Isd) of the stator current (Is) needed to maintain the desired flux profile
in the motor. The inputs are Flux Feedback and Stator Frequency from the motor model, Speed
Feedback and Torque Reference from the speed control block and the measured voltage at the
input of the bridge Vline Bridge. The Flux Feedback is subtracted from the Flux Reference to
determine the Flux Error, which is the input to the flux PI regulator. The gains are determined
from desired Flux Regulation Bandwidth and motor parameters T Rotor and L Magnetizing. The
output of the flux regulator is Isd Command 1. An open loop estimate of the magnetizing current,
Isd Command 0 is determined by dividing the Flux Reference by parameter L Magnetizing. Isd
Command 0 and Isd Command 1 are added to produce Isd Command which is the magnetizing
component of the stator current command. To calculate the magnetizing current supplied by the
inverter Ix Command (312), the current supplied by the motor filter capacitor in magnetizing is
calculated and subtracted from Isd Command. Iy Command from Speed Control block and Ix
Command are then passed to the Current Control block to determine the dc link current reference
(Idc Reference) and the firing angles of the two converters (Alpha Line and Alpha Machine). The
flux profile in the drive is adjusted by the parameters Flux Command No Load and Flux
Command Base Speed. Using these parameters, Flux Reference is adjusted linearly with the
desired Torque Reference. At light loads motor flux is decreased allowing reduction in losses
while full flux is produced at rated load. The maximum flux reference is limited to Flux
Command Limit. This limit depends on the input voltage Vline Bridge and the motor speed
(Speed Feedback). If the drive operates at reduced line voltage, then Flux Reference is reduced.
Also if the motor is running above the Base Speed, the flux profile is made inversely proportional
to the speed of the motor resulting in the field weakening and the constant power mode of
operation of the drive. This is accompanied by a decrease in the motor torque capability.
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FUNCTIONAL DESCRIPTION OF THE DRIVE
Figure 3 – Flux control block
2.4. Current control block
The function of the current control block, which is shown in Figure 4, is to determine the
firing angles for the converters Alpha Line and Alpha Machine. The inputs are the torque (Iy
Command) and flux producing (Ix Command) components of the dc link current command from
the speed control and flux control blocks respectively, and the measured dc link current Idc
Feedback. The square root of the sum of the squares of Ix Command and Iy Command determines
the dc link current reference Idc Reference. This is subtracted from the measured dc current
feedback to determine Idc Error. This is processed by the current regulator to produce Vdc Error.
To effectively control the dc link current an estimate of the motor side dc link voltage is done to
calculate Vdc Feed Forward which is added to Vdc Error to produce the reference voltage for the
line side converter Vdc Reference. The line converter firing angle is the inverse cosine of Vdc
Reference. The machine converter firing angle is determined by taking the inverse tangent of the
ratio of Iy Command to the Ix Command. The quadrant of operation is adjusted based on the signs
of the current commands.
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FUNCTIONAL DESCRIPTION OF THE DRIVE
Figure 4 – Current control block
2.5. Line converter feedback block
The function of the line converter feedback block is to process (scale and filter) the line
side voltage and current feedback signals before being sampled by the drive control software. The
line converter feedback block provides a total of six voltage feedback signals representing the
three ac (Va1, Vb1, and Vc1), two dc (Vdc+, Vdc-) and one line side filter capacitor voltages
referenced to ground. The three line-to-ground voltages are subtracted from each other to produce
the three line to line voltages (Vab, Vbc and Vca). These line voltages are filtered and sampled by
software for synchronization and protection. The two dc voltages are subtracted to determine the
line side dc link voltage (Vdc), which is used for hardware dc link over-voltage protection. In
PWM drives, the neutral point of the line filter capacitor is measured and used for line side
neutral over-voltage protection. Current transformers in two of the ac input lines provide the
input line current feedback (Ia, Ic,). These currents are then filtered and processed by a variable
gain stage. Inverting and adding the two current feedback signals reproduce the current in the
remaining phase (Ib). Moreover, the average value of the dc link current feedback is measured
using a V-f converter and used by the dc link current controller to calculate the firing angle for the
rectifier. Figure 5 shows the rectifier, DC link and the inverter.
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FUNCTIONAL DESCRIPTION OF THE DRIVE
Figure 5 – Rectifier, DC link and inverter
2.6. Machine converter feedback block
The function of the machine converter feedback block is to process (scale and filter) the
raw voltage and current feedback signals to the form required by the drive control software. The
machine converter provides a total of six voltage feedback signals representing the three ac (Va1,
Vb1, Vc1), two dc (Vdc+, Vdc-) and one machine side filter capacitor neutral voltage referenced to
ground. The motor line to ground voltages are subtracted from each other and filtered to produce
the three motor line to line voltages (Vab1, Vbc1, Vca1). The two dc voltages are subtracted to
determine the machine side dc link voltage (Vdc), which is used for hardware dc link over-voltage
protection. The motor line-ground voltages are summed to determine the motor neutral to ground
voltage (Vng) and are used for motor neutral over-voltage protection. Machine Converter provides
stator current feedback in two of the motor phases (Ia3-out, Ic3-out). These currents are then filtered
and processed by a variable gain stage (Ia3, Ic3) before being sampled for protection. Inverting and
adding the two current feedback signals reproduces the current in the remaining phase (Ib3). The
motor line voltages and currents are further used to calculate the motor flux (Fab, Fbc, Fca) using a
hardware analog model. The measured flux (Vd and Vq) is then used in the motor model block for
synchronization and drive control.
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FUNCTIONAL DESCRIPTION OF THE DRIVE
2.7. Motor model block
The function of the motor model block, which is shown in Figure 6, is to determine the
rotor flux position (Flux Angle), flux feedback (Flux Feedback), applied stator frequency (Stator
Frequency), slip frequency (Slip Frequency) and motor operating variables like stator current
(IStator), stator voltage (VStator), torque (Torque), power (Motor Power) and power factor (Motor
Power Factor). The PowerFlex7000 uses Rotor Flux oriented control to achieve independent
control of motor flux and torque. This is achieved by synchronizing the machine converter gating
to Flux Angle. To determine the flux feedback, stator frequency and the synchronizing reference
frame the drive uses either the Voltage or the Current model. For speeds greater than 3Hz, the
drive uses the voltage model (hardware analogue flux model) to calculate the Flux from Voltage,
Flux Angle from Voltage and Stator Frequency from Voltage. Below 3Hz, the drive uses the
current model to calculate Flux from Current, Flux Angle from Current and Stator Frequency
from Current. The current model is based on indirect vector control and uses the d-q components
of stator current along with motor parameters T Rotor and L Magnetizing. Based on the operating
speed of the drive and the speed feedback mode (Sensorless or Pulse Tachometer), a flux select
algorithm determines the model to be used. Motor model also calculates the Slip Frequency
which is used in the calculation of the motor speed (Speed Control) in Sensorless mode and for
determining the rotor flux position in Pulse Tachometer mode. The synchronously rotating frame
(Flux Angle) is used in transforming the measured motor currents and voltages into d-q
components. The direct axis components (Isd and Vsd) are in phase with the rotor flux, while the
quadrature axis components (Isq and Vsq) are displaced 90 degrees from the rotor flux. The stator
current (IStator) and voltage magnitudes (VStator) are calculated by taking the square root of the sum
of the squares of the respective d-q components. The motor Torque is calculated by multiplying
the Flux Feedback and Isq with motor torque constant. Torque multiplied by the motor speed gives
the Motor Power. Motor Power Factor is determined as the ratio of motor active power and the
apparent power.
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VECTOR CONTROL
Figure 6 – Motor model block
3. Vector control
In order to fully understand the concept of vector control it is important to be familiar
with various transformations and with mathematical model of the induction machine. Afterwards,
the idea of vector control is very straightforward.
3.1. Transformations used in vector control
In order to implement vector control it is necessary to transform currents and voltages
from three-phase stationary coordinate system of stator into rotating dq coordinate system of
rotor flux. The main reason is that such a transformation simplifies calculation, and therefore,
makes Vector control feasible in real time. This transformation could be split into two steps:
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VECTOR CONTROL
1) Transformation from three-phase stationary system into two-phase (also
calledαβ ) stationary coordinate system. This transformation is also known as Clark
transformation
2) Transformation from two-phase coordinate system into rotating dq coordinate
system. This transformation is called Park transformation.
3.1.1. Clark transformation
In general, the vector in the two-phase coordinate system (Figure 7) could be described as
follows:
)( 2cba xaxaxkx ++=
)(00 cba xxxkx ++=
jeaj
235.03
2
+−==π
jeaj
235.03
22 −−==
− π
{ } ( )[ ]cba xxxkx +−= 5.0Re
{ } ( )cb xxkx −=23Im
Since we want the transformation to fulfill following condition { } 0Re xxxa += , we can estimate
k and k0 as follows:
{ } { } =⎭⎬⎫
⎩⎨⎧
−−++−+=++= ))235.0()
235.0((Re)(ReRe 2
cbacba xjxjxkxaxaxkx
0
0
0
0 5.05.15.0))(5.0(kx
kkxxkx
kkxxxxk aaacba −=⎟⎟⎠
⎞⎜⎜⎝
⎛−−=+−=
{ } 0Re xxx a −= , therefore 3215.1 =⇒= kk and
3115.0 0
0
=⇒= kkk
Finally, the transformation is defined as follows:
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VECTOR CONTROL
{ } ( )
{ } ( )
)(31
31Im
31
32Re
0 cba
cb
cba
xxxx
xxx
xxxx
++=
−=
+−=
(1a-1c)
Or alternatively by means of matrix notation:
( ) ( )
⎟⎟⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜⎜⎜
⎝
⎛
−−
−=
21
23
21
21
23
21
2101
32
cba xxxoβα (2)
Inverse transformation is defined as follows: { }
{ } { }
{ } { } 0
0
0
Im23Re
21
Im23Re
21
Re
xxxx
xxxx
xxx
c
b
a
+−−=
++−=
+=
(3a-3d)
Or alternatively by means of matrix notation:
( ) ( )
⎟⎟⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜⎜⎜
⎝
⎛
−
−−
=
11123
230
21
211
oxxx cba βα (4)
Figure 7 – Vector in two-phase coordinate system
27
VECTOR CONTROL
3.2.1 Park transformation
The idea of the vector control is that we control the relative position between the rotor
flux and the stator current. In order to achieve this, we need one coordinate system for both flux
and current. It has been proven that equations become much simpler if we use dq rotation system
of rotor flux. Equations which described Park transformation could be easily obtained from
Figure 8:
Figure 8 – Park transformation
( ) ( )( ) ( )
αβ
φαφβφβφα
ooqd
dq =−=+=
sincossincos
(5a-5c)
Or alternatively by means of matrix notation:
( ) ( )( ) ( )( ) ( )
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛ −=
1000cossin0sincos
φφφφ
βα αβooqd dq (6)
Where φ is electrical angle (theta) which is angle between dq coordinate system and two-
phase (αβ ) stationary coordinate system.
28
VECTOR CONTROL
It also should be noted that since the origin of all coordinate systems is the same and that
we assume a balanced three-phase system (xo= 0) then: 00 === xoodq αβ
Inverse transformation is defined as follows: ( ) ( )( ) ( )
dqooqdqd
=+=−=
αβ
φφβφφα
cossinsincos
(7a-7c)
Or alternatively by means of matrix notation:
( ) ( )( ) ( )( ) ( )
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛−=
1000cossin0sincos
φφφφ
βα αβ dqoqdo (8)
3.2.2 Mathematical model of induction machines
A dynamic model of induction machine subjected to control must be known in order to
understand and design vector controlled drive. Due to the fact that every good control has to face
any possible change of the plant, it could be said that the dynamic model of the machine could be
just a good approximation of the real plant. Nevertheless, the model should incorporate all the
important dynamic effects occurring during both steady-state and transient operations. The
equivalent electrical circuit diagram of one phase of induction (asynchronous) machine is shown
in Figure 9.
Figure 9 – Equivalent circuit diagram of one phase of an induction machine
First of all, equations which hold for induction machines are:
29
VECTOR CONTROL
msls LLL += (9)
( ) msslmmsslrsmsslrmsss iLiLiLiiLiLiLiL ψψ +=+=++=+= (10)
mrlr LLL += (11)
( ) mrrlmmrrlrsmrrlsmrrr iLiLiLiiLiLiLiL ψψ +=+=++=+= (12)
m
mmrs L
iii ψ==+ (13)
Substituting mψ from (9) into (10) we get:
rrlssirs iLiL −+=ψψ (14)
Substituting mψ from (12) into (14) we get:
rm
rl
m
rsr i
LL
Lii −+−=
ψ (15)
Substituting from (11) into (15) we get: rlL
rr
sr
mr L
iLLi ψ1
+−= (16)
Substituting from (16) into (14) we get: ri
srlr
mslr
r
mr
r
rls
r
mrlssirrrlssirs iL
LLL
LL
LLi
LLLiLiLiL ⎟⎟
⎠
⎞⎜⎜⎝
⎛++=−
+++=−+= ψψψψψ (17)
Now let us first assume three coordinate systems:
I) Stator coordinate system
Axis: x,y
Velocity: 0=Iω
II) Rotor coordinate system
Axis: x,y
Velocity: mII pωω =
Where p is the number of pair of poles and mω is the angular velocity of the rotor
III) Rotor flux coordinate system
Axis: dq
Velocity: ψωω =III
30
VECTOR CONTROL
Where ψω is the angular velocity of the rotor flux.
The equations, which describe the ac drive (Figure 8), are:
a) Stator equations:
dtd
iRu
dtd
iRu
dtd
iRu
scscssc
sbsbsba
sasassa
ψ
ψ
ψ
+=
+=
+=
(18a-18c)
Where isa, isb and isc are stator currents, usa, usb and usc are stator voltages and saψ , sbψ and
scψ are stator fluxes. And hence, the space vector in the stator coordinate system is:
( )
( )dt
diRuuuku
dtdiRuauauku
sssscsbsas
sIsIsscsbsasI
0000
2
ψ
ψ
+=++=
+=++= (19a-19b)
Since ( ) ( ) 0000 ssscsbsasscsbsas iLiiiLkk σσψψψψ =++=++= , if then 00 =si 00 =su
b) Rotor equations:
dtd
iRu
dtd
iRu
dtd
iRu
rcrcsrc
rbrbsrb
rarasra
ψ
ψ
ψ
+=
+=
+=
(20a-20c)
Where ira, irb and irc are rotor currents, ura, urb and urc are rotor voltages and raψ , rbψ and
rcψ are rotor fluxes. And hence, the space vector in the rotor coordinate system is:
( )
( )dt
diRuuuku
dtdiRuauauku
rrrrcrbrar
rIIrIIrrcrbrarII
0000
2
ψ
ψ
+=++=
+=++= (21a-21b)
Since ( ) ( ) 0000 rrrcrbrarrcrbras iLiiiLkk σσψψψψ =++=++= , if then 00 =ri 00 =ru
31
VECTOR CONTROL
In order to transform above equations into rotor flux coordinate system it is important to
realize that:
a) Transformation from coordinate system I to coordinate system II: εj
III exx =
Where Ix and IIx are space vectors in the coordinate system I and II, respectively and τ
is angle between coordinate system I and coordinate system II.
b) Transformation of derivatives from coordinate system I to coordinate system II:
Equation in the coordinate system I:
dtdyx I
I =
Equation in the coordinate system II:
( ) ωεεεεε jydtydx
dtdjeye
dtydey
dtdex II
IIII
jII
jIIjII
jII +=⇒+==
Where ω is a relative angular velocity between the coordinate system I and II.
So now we can transform equations (19a and 21a) into the rotor flux coordinate system:
( ) rIIImrIII
rIIIrrIIIIIIIIrIII
rIIIr
sIIIsIII
sIIIssIIIIIIsIII
sIIIssIII
pjdt
diRjdt
diR
jdt
diRjdt
diRu
ψωωψψωψ
ψωψψωψ
ψ
ψ
−++=++=
++=++=
,0 (22a-
22b)
Since for the squirrel-cage 0=rIIIu .
Where υ is the angle between the coordinate system I and III.
IIIdtd ωυ
= is a angular speed of the rotor coordinate system
Where ε is the angle between the coordinate system II and III.
IIIIIdtd
,ωε= is a relative angular speed between the rotor flux coordinate system and the rotor
coordinate system.
Substituting ψ
ψ
ωωω mp
s−
= we get:
32
VECTOR CONTROL
rIIIrIIIrIIIr
sIIIsIII
sIIIssIIIIIIsIII
sIIIssIII
jsdt
dss
iR
jdt
diRjdt
diRu
ψωψ
ψωψψωψ
ψ
ψ
++=
++=++=
10 (23a-
23b)
Substituting sψ from (17) into (22a) yields:
srlr
mslIII
srlr
mslr
r
m
sIIIssIII iLLLLj
dt
iLLLL
LLd
iRu ⎟⎟⎠
⎞⎜⎜⎝
⎛++
⎥⎦
⎤⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛++
+= ωψ
(24)
Complex equations (24 and 22b) in dq coordinate system (rotor flux coordinate system) are:
( )
( ) rdmrq
rqr
rq
r
mr
rqmrd
rdr
rs
r
mr
sdrlr
mslrd
r
msqrl
r
msl
rq
r
msqssq
sqrlr
mslrq
r
msdrl
r
msl
rd
r
msdssd
pdt
dLRi
LLR
pdt
dLRi
LLR
iLLLL
LL
dtdi
LLLL
dtd
LLiRu
iLLLL
LL
dtdiL
LLL
dtd
LLiRu
ψωωψ
ψ
ψωωψψ
ωωψψ
ωωψψ
ψ
ψ
ψψ
ψψ
−+++=
−+++=
⎟⎟⎠
⎞⎜⎜⎝
⎛++−⎟⎟
⎠
⎞⎜⎜⎝
⎛+++=
⎟⎟⎠
⎞⎜⎜⎝
⎛++−⎟⎟
⎠
⎞⎜⎜⎝
⎛+++=
0
0
(
(
(25a-25d)
Equations (25a-25d) are in rotor flux coordinate system which implies:
rrdrq
rq dtd
ψψψ
ψ =⇒=⇒= 00
Hence, equations (25a-25d) could be simplified as follows:
( ) rdmsqr
mr
rdrd
r
rsd
r
mr
sdrlr
mslrd
r
msqrl
r
mslsqssq
sqrlr
msl
sdrl
r
msl
rd
r
msdssd
piLLR
dtd
LRi
LLR
iLLLL
LL
dtdi
LLLLiRu
iLLLL
dtdiL
LLL
dtd
LLiRu
ψωω
ψψ
ωωψ
ωψ
ψ
ψψ
ψ
−+=
++=
⎟⎟⎠
⎞⎜⎜⎝
⎛++−⎟⎟
⎠
⎞⎜⎜⎝
⎛++=
⎟⎟⎠
⎞⎜⎜⎝
⎛+−⎟⎟
⎠
⎞⎜⎜⎝
⎛+++=
0
0
( (26a-26d)
Realizing that torque could be defined as follows:
33
VECTOR CONTROL
{ } ( ) mm
sdrqsqrdr
mrr
r
me F
dtdJTiip
LLijp
LLT ωωψψψ ++=−==
23Re
23 *
(27)
Where T is torque of the load on the shaft, Te is electromagnetic torque on the shaft
produced by motor, J is inertia of the motor, F is friction factor of the motor and p is pole pairs.
Then (26a-26d) could be expressed in state space:
CxyBuAxx
=+=&
Letting:
r
mrlsl L
LLL +=λ
λα
2
2
r
mrs L
LRR +
=
λβ
2r
mr L
LR
=
pLL
r
m
λγ
2
=
λδ 1=
pJL
L
r
m 123
=ε
pFL
L
r
m 123
=χ
isd=x1, isq=x1, 3xrd =ψ , 4xrq =ψ , 5x=ω , usd=u1, usq= u2, T=u3, Te=y1,
we get:
34
VECTOR CONTROL
⎟⎟⎟
⎠
⎞
⎜⎜⎜
⎝
⎛
⎟⎟⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜⎜⎜
⎝
⎛
−
+
⎟⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜⎜
⎝
⎛
⎟⎟⎟⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜⎜⎜⎜
⎝
⎛
−
−−
−−−
=
⎟⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜⎜
⎝
⎛
3
2
1
5
4
3
2
1
34
5
5
5
5
5
4
3
2
1
1000000000000
00
00
00
0000
uuu
Jxxxxx
xxLRx
LRR
xLR
LL
R
xx
xxxxx
m
r
m
rr
m
r
r
mr
δδ
χεε
βγαγβα
&
&
&
&
&
(28)
⎟⎟⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜⎜⎜
⎝
⎛
⎟⎟⎠
⎞⎜⎜⎝
⎛−=
5
4
3
2
1
541 00023
23
xxxxx
pxLL
pxLL
yr
h
r
h (29)
Equation (28) fully describes mathematical model of induction machine and in following
sections will be used to derive the concept of Vector control.
3.2.3 Mathematical concept of Vector control and relation to the control of DC machines
Since one may be familiar with the control of DC machines, I will introduce Vector
control in relation to the control used in DC machines.
The construction of a DC machine is such that the field flux is perpendicular to the
armature flux. Being orthogonal, these two fluxes produce no net interaction on one
another. Adjusting the field current can therefore control the DC machine flux, and the
torque can be controlled independently of flux by adjusting the armature current.
Equations describing DC machines are:
( ) ( )τ
τψ
ψψ
ssUs
ILIkT
m
mm
a
+=
==
1
(30a-30c)
Where Ia is armature current, ψ is flux in the motor, Im is field (magnetizing) current, Um
is field voltage and τ is time constant of DC motor.
An AC machine is not so simple because of the interactions between the stator and
the rotor fields, whose orientations are not held at 90 degrees but vary with the operating
conditions. You can obtain DC machine-like performance in holding a fixed and
35
VECTOR CONTROL
orthogonal orientation between the field and armature fields in an AC machine by
orienting the stator current with respect to the rotor flux in order to attain independently
controlled flux and torque. In order to achieve this it is important to split stator current
into two parts:
a) Component which is aligned with rotor flux and which produces magnetic field in the
motor - Isd
b) Component which is perpendicular to rotor flux and produces torque - Isq
Equations (28) fully described the induction machine. In order to control both Isq and
Isd it is necessary to estimate the position and magnitude of rotor flux. So the magnitude
could be estimated from (28) or rather from (26c):
( ) ( )sis
Ls sd
r
mr τ
ψ+
=1
(31)
Where r
rr R
L=τ is the rotor time constant and rdr ψψ = because we use dq coordinate
system where d axis is aligned with rotor flux.
The position could be estimated from (28) or rather from (26d):
msqrr
mrmsq
rdr
mr pi
LL
RpiLL
R ωψ
ωψ
ωψ +=+=11 (32)
Now it is obvious that vector control is fairly similar to the control of DC machines:
DC Machine AC Machine
aIkM ψ= sqrd IkM ψ=
( ) ( )τ
τψs
sUs m +=
1 ( ) ( )sI
sL
ss sdm
rdr τψψ
+==
1)(
Table 2 – Comparison of the control of DC and AC motors
Isq in induction machines corresponds to armature current Ia in DC machines. However, in
terms of dynamics Isd in induction machines corresponds to field voltage Um in DC machines.
Nonetheless, the control of the stator current is achieved by the control of the stator voltage. So,
all in all, we shall say that Vector control is conceptually the same as the control of DC machines.
36
VECTOR CONTROL
3.2.4 Vector control in the nutshell
Let us summarize the concept of Vector control. The Vector control, which basic principle
is shown in Figure 10, could be described in following steps:
1) Transforming stator currents ia, ib, ic from three-phase stator stationary
coordinate system into dq rotating rotor flux coordinate system. This step
performed by Clark and Park transformation according to (2) and (4),
respectively.
2) Finding new position of the rotor flux using isq, rotor angular speed mω and
magnitude of the rotor flux rψ . This is calculated by (32).
3) Finding the new magnitude of the rotor flux using isd. This is calculated by
(31).
4) Setting new value of id according to the flux reference and magnitude of the
flux profile in the motor. The relation between flux and isd is:
msd L
i ψ= (33)
5) Setting new value of iq according to the flux reference and the torque which is
adjusted according to the speed error This is calculated by:
ψM
LL
pi
m
rsq
132
= (34)
6) Transforming id and iq from dq rotating rotor flux coordinate system into three-
phase stator stationary coordinate system using Inverse Park and Clark
transformation. This is done by (6) and (8), respectively.
It should be noted that the Vector control hugely depends on knowing parameters of the
motor. However, when the Vector control is used in close-loop speed regulation, the uncertainties
of motor parameters could affect only the dynamics of the motor but not the accuracy since the
torque is adjusted according to speed error.
37
VECTOR CONTROL
Figure 10 – The basic principle of Vector control
38
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
4. Implementation of PowerFlex7000 in Simulink
The model, which is shown in Figure 11, is divided into 8 main parts. First five parts
functionally and logically correspond to its counterparts of the Frequency Converter
PowerFlex7000. However, some changes have been made in order to improve the simulation and
to make the model easier implement. These changes are discussed in detail at the end of this
chapter.
Figure 11 – High-level scheme of the model in Simulink
39
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Input Function and Description Torque It defines load on the motor shaft SPs Set point for speed SPt Set point for torque Synchro Reg “Flying start”
Table 3 – List of inputs of Frequency Converter
Output Function and Description Speed-info Information regarding speed variables Flux-info Flux variables Motor model-info Motor model variables Current-info Current variables Motor-info Motor-induction machine variables Speed Speed of the motor [rad/s]
Table 4 – List of outputs of Frequency Converter
Parameter Sample time [s]1
Table 5 – List of parameters of Frequency Converter
4.1. Speed Control block
The Speed control block determines the torque-producing component of the stator current.
This block provides four modes of operation:
1) Speed regulation
Speed control block in the speed regulation determines the torque-producing current. First
of all, speed reference is processed by Speed Reference block, where the reference speed
is being clamped to minimal and maximal value and its rate of change is also limited. This
should prevent drive from oscillating when the load on the shaft of the motor is low and,
therefore, there is possibility that the motor could accelerate too fast and there is a danger
of motor being damaged. Alternatively, it could define the start-up speed characteristic.
Next input to this block is speed feedback, which is filtered, then is subtracted form speed
reference to produce speed error. Subsequently, speed error is processed by PI Speed
1 This parameter should be set exactly the same as the Sample time of the simulation (Simulation->Configuration Parameters->sample time -when using discrete solver) or the sample time in GUI SimPowerToolbox
40
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
regulator which adjusts torque according to speed error. Finally, Speed control block
determines Iq command and passes command to Motor model block. Calculating the
current is done according to (34).
2) Torque regulation
PI Speed regulator is being by-passed and requested torque defines Iq according to (34).
Since the regulating of torque is done in open loop, the accuracy is much less precise than
the regulation of the speed. Requested torque is passed to Speed control block via Torque
Command External.
3) Speed sum
In this mode of operation reference speed is obtained from two different sources (Speed
Reference and Torque command External) and then is summed up and controlled as
described in Section 1.
4) Zero torque
This mode enables tuning drive since the input is zero.
Another input is Magnetizing, which defines normal and magnetizing modes of
operation. When this input is set to zero, motor needs to be magnetized further. Whereas,
magnetizing equals one indicates normal mode of operation, which means one of
abovementioned operation.
Next input to this block is Synchronized Regulation Output. The function of this
signal is to speed up or slow down the motor from outside when the drive is to be shut
down or when the motor is to be control from different source (e.g. another frequency
converter). This so-called flying start could come in handy when the frequency converter
needs to be serviced and the motor must run continually. This provides a smoother, safer
and, moreover, predictable reaction.
Last input to this block is Flux Reference which is used in calculating Isq command
according to (34).
41
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
The only output of this block (with the exception of Isq Command and Monitor) is
Torque Reference, which is passed to Flux Control block to set Flux Reference.
Last block which is implemented in this block is “Detect start”. This block, which
is shown in Figure 13, emulates the situation when the converter is being turned on for the
first time, but stand still state is required. Therefore, this block disables 6-pulse
Synchronized Generator (part of Line Side Converter Gating and Feedback) and also
prevents Flux Control block from starting magnetizing mode. When the regulation is
required, the change is detected and 6-pulse Synchronized Generator is unlock and
magnetizing operation of the motor is enabled.
Figure 12 – Speed Control block in Simulink-
Figure 13 – Detect Start Control block in Simulink
42
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Input Function and Description
Speed Feedback Set point for speed Magnetizing Determines normal mode of operation Synchronized Regulation Output Used in “Flying Start” Speed Feedback Signal from tachometer Torque Command External Set point for torque Flux Reference Signal from Flux Control block
Table 6 – List of inputs of Speed Control block
Output Function and Description Torque Reference Desired torque Isq Command Desired torque-producing current Monitor Monitoring Speed Control block variables Run Determines start of the drive
Table 7 – List of outputs of Speed Control block
Parameter Regulation type Speed controller - proportional gain Speed controller - integral gain Speed measurement - low-pass filter cutoff frequency [Hz] Controller output torque saturation [N.m] [negative, positive] Torque rate limit [N.m/s] [negative, positive] Mutual inductance [H] Rotor leakage inductance [H] Motor pairs of poles Maximal speed [rpm] Minimal speed [rpm] Speed ramp [rmp/s] [negative, positive]
Table 8 – List of parameters of Speed Control block
4.2. Flux Control block
The main aim of flux Control block, which is shown in Figure 13 is to determine the flux-
producing stator current Id. This is achieved by setting the reference flux. Reference flux depends
on desired torque, which is passed from Speed Control block and on parameters Initial flux,
Nominal flux and Nominal torque. The desired reference flux is linearly proportional to torque
(Figure 14) and it is adjusted in Flux Table block (Figure 16). When the drive operates under
Base speed of the motor or at reduced line voltage then Nominal flux is reduced by Flux
Command Limit block (Figure 17). Next stage is to determine flux command according to flux
error this is achieved by PI Flux Regulator. The final step is to determine Isd command which is
43
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
done by dividing Flux command by Mutual inductance (Lm). Isd command is then pass to Motor
model to determine absolute value of Istator and angle between Istator and rotor flux.
Figure 14 – Flux Control block in Simulink
Figure 15 – Flux reference
Figure 16 – Flux Table block in Simulink
44
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Figure 17 – Flux Command Limit block in Simulink
Input Function and Description Run Determine start of the drive Speed Feedback Signal from tachometer VLine Average Input voltage of Frequency Converter Toqrue Reference Torqe Reference form Speed control Flux Feedback Estimation of Flux from Motor model
Table 9 – List of inputs of Flux Control block
Output Function and Description Flux Reference Desired flux profile Isd Command Desired fux-producing current Monitor Monitoring Flux Control block variables
Table 10 – List of outputs of Flux Control block
Parameter Speed feedback – low-pass filter cut-off frequency [Hz] Flux controller - flux estimation low-pass filter cut-off frequency [Hz] Flux controller - proportional gain Flux controller - integral gain Flux controller - flux limit [Wb] [negative, positive] Nominal torque [Nm] Base speed [rpm] Initial flux [Wb] Nominal flux [Wb] Mutual inductance [H] Rated line voltage [V]
Table 11 – List of parameters of Flux Control block
45
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
4.3. Current Control block
The aim of this block is to set Ia ,Ib and Ic stator currents as required from Motor model
block. The currents are passed from motor model in xy (αβ ) stator stationary coordinates. The
absolute value of the desired current is controlled by changing firing angle of the Line side
rectifier, whereas the frequency of the current is set by changing the gating sequence of the
inverter. This block is shown in Figure 18.
a) Control of Idc:
Idc feedback is filtered and subtracted from the absolute value of desired
current. However, since Idc and absolute value of desired current is related as
follows:
statorDC II32
π=
Absolute value must be reduced before being subtracted. Then PI Current
regulator processes Idc error and produces Vdc voltage. Nonetheless, in order to
control Idc effectively feedforward voltage is filtered and divides by VLine
Average, which is the absolute value of voltage before the rectifier. Then voltage
is limited by Current limiter in order to limit current. It should be noted that
voltage could be reduced only relatively to the input voltage. The desired voltage
is then processed by Cos-1 block. This block determines Alpha line angle by using
arcos of desired output voltage. Since output voltage tends to oscillate, the firing
angle could be limited. Limiting the firing angle could improve the regulation of
current significantly. However, sometimes it slows down the response, especially
when the reference current drops rapidly down.
b) Control of frequency of stator voltage
The frequency of stator voltage is control by changing the rate of inverter
sequence. The scheme of vector current modulator is shown in Figure 18. First of
all, the desired angle of the current is adjusted by PI Phase regulator. This is the
result of the fact that the combined electrical circuit of the inverter and the motor
is frequency dependent. So, in fact, the reference current is phase shifted. And
46
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
vector control is based on the idea that the stator current is precisely control
relatively to the rotor flux. In another words, stator current must be synchronized
accurately to rotor flux. Therefore, it is important to maintain the desired angle. If
this control was not the vector one, the phase shift would not be an issue and the
only difference would be that the dynamic response would be slightly slower. The
adjusted reference phase is than divided into 6 sectors each having 60° in Sector
Selector. Switching time calculator block is each cycle triggered (this block
processes only when is triggered) by Ramp Generator. This time is determined by
switching frequency of vector modulator. On-times and off-times for 6 switches of
the inverter are determined in Switching Time Calculator block. The idea is
straightforward (Figure 20):
7) In each cycle only two switches are in on-state
8) In each cycle only two switches are modulated and in such a way that on-time
of one switch is 0 and off-time depends on the reference sin wave, which has
unit amplitude , the other switch has its off-time set to on-time of the first
switch and off-time set to1.
9) Between on and off-times some short delay (death time) must be placed due to
commutation.
10) The change of current flow is achieved by switching upper and lower switches.
The final stage is to compare on and off-times with unit ramp and turn on or off
corresponding switches. Figure 16 shows the principle of this modulation. In order
to demonstrate the modulation, there were no inductances in the circuit so the
current is not smooth and also the frequency was chosen to be much lower than in
the model. Hence, the parts which are not modulated are quite wide compared to
parts which are modulated.
In order to get almost maximal breakaway torque it is necessary to reach certain
level of magnetic saturation. This level is determined by Motor model and prior to
normal operation the vector modulator is by-passed and the current flows through
2 phases.
47
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Figure 18 – Current control block in Simulink
Figure 19 – Vector modulator in Simulink
48
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Figure 20 – PWM modulation – the principle
Input Function and Description Magnetizing Determine the start of normal operation Phase Feedback Phase of stator voltage Xy Desired stator current Idc Current feedback from DC link Stator Voltage Stator voltage in ab coordinates VLine Average Voltage before the rectifier
Table 12 – List of inputs of Current Control block
Output Function and Description Alpha Machine Firing angle for the Inverter Alpha Line Firing angle for the Rectifier Monitor Monitoring Current Control variables
Table 13 – List of outputs of Current Control block
Parameter Current controller - proportional gain Current controller - integral gain Relative current limit [negative, positive] Current rate limit [negative, positive] Current feedback – low-pass filter cut-off frequency [Hz] Voltage feedback – low-pass filter cut-off frequency [Hz] Phase feedback – low-pass filter cut-off frequency [Hz] Phase controller - proportional gain Phase controller - integral gain Phase controller - phase limit [rad] [negative, positive] Switching frequency of vector modulator [Hz]
Table 14 – List of parameters of Current Control block
49
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
4.4. Motor model
Motor model, which is shown in Figure 21, transforms stator currents ic, ib and ic from
three-phase stationary stator coordinate system into dq rotating system of the rotor flux. This is
exactly done by (2) and (4). Subsequently, this block estimates the magnitude of rotor flux and
position this is done according to (31) and (32), respectively. Then the Motor model processes Iq
and Id Commands from Speed Control block and Flux Control block, respectively, and transforms
these currents from dq rotating system of rotor flux into three-phase stationary stator coordinate
system using (6) and (8). Afterwards, Motor model passes desired current to Current control
block. Another output to Current control block is Phase Feedback which is phase of the stator
current. This signal is then used by PI Phase regulator to adjust the reference phase in order to
match desired phase. Block Magnetizing detects when the flux profile in the motor reaches
certain level which is user-defined.
Figure 21 – Motor model
50
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Input Function and Description Iabc Stator current Speed Feedback Signal from tachometer Isd Command Desired flux-producing current Isq Command Desired torque-producing current
Table 15 – List of inputs of Motor model block
Output Function and Description Flux feedback Estimation of magnitude of rotor flux Magnetizing Start the normal mode of operation Phase Feedback Phase of stator current Monitor Monitoring Motor model variables
Table 16 – List of outputs of Current cntrol block
Parameter Motor mutual inductance (H) Motor rotor resistance (ohms) Motor rotor leakage inductance (H) Motor pairs of poles Magnetizing flux (Wb)
Table 17 – List of parameters of Motor model block
51
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
4.5. Line side converter gating and feedback
Line side converter, which is shown in Figure 22, includes two standard blocks from
SimPowerToolbox:
a) Synchronized 6 –Pulse Generator
This block is always in double-pulsing mode
b) Thyristors Rectifier
This block processes firing angle from Current Control block and, hence, controls
indirectly the stator current.
The only output from this block is Vline Average, which is used in Flux Control block to
determine flux profile in the motor and in the Current Control block to effectively control DC
link current.
Figure 22 – Line side converter gating and feedback
52
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Input Function and Description Alpha Line Determines the firing angle for rectifier A,B,C,N A,B,C phase and neutral wiring
Table 18 – List of inputs of Line side converter gating and feedback
Output Function and Description VLine Average Voltage at the input of the Converter IDC DC Link current +,- DC Voltage
Table 19 – List of outputs of Line side converter gating and feedback
Parameter Synchronizing frequency of rectifier [Hz] Pulse width of rectifier [deg] Snubber resistance of thyristors [ohm] Snubber capacitance of thyristors [F] On-state resistance of thyristors [ohm]
Table 20 – List of parameters of line Side converter gating and feedback
4.6. Motor side converter gating and feedback
Machine Side Converter Gating and Feedback, which is shown in Figure 23, provides the
converter with reference stator currents ia, ib and ic. These currents are further used in Motor
model block in estimating the magnitude and the position of the rotor flux. The phase is also
detected in order to synchronize the inverter with the reference current. Another output is the
stator voltage which is used in the Current Control block to effectively control DC link current.
The next input is Alpha Machine. This signal is passed from Current Control block and it controls
Three-phase Inverter. Gating is passed as a vector containing always six numbers each
corresponding to one switch (phase A-upper, phase A-lower, phase B-upper, phase B-lower,
phase C-upper, phase C-lower). Each vector always contains 4 zeros or more (more when the
Vector Modulator uses death times due to the commutation). Number 1 indicates on-state of the
switch. Whereas number 0 indicates off-state of the switch. Since Powerflex7000 maintains
SGCT (Symmetrical Gate Commutated Thyristors), which is rather new silicon device and it is
not a part of SimPowerToolbox, I modeled such an device as a ideal switch (with non-zero
resistance in on-state) with RC snubber circuit. The inverter is shown in Figure 24.
53
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Figure 23 – Motor side converter gating and feedback
Figure 24 – Three-phase inverter
54
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Input Function and Description
Alpha Machine Gating for the inverter +,- DC Voltage
Table 21 – List of inputs of Machine side converter gating and feedback
Output Function and Description Iabc Stator currents Mta,Mtb,Mtc Motor wirings Stator Voltage Stator Voltage
Table 22 – List of outputs of Machine Side converter gating and feedback
Parameter Snubber resistance [ohm] Snubber capacitance [F] On-state resistance [ohm]
Table 23 – List of parameters of Machine side converter gating and feedback
4.7. Differences between the model and PowerFlex7000
The model differs in some way; however, the performance of the model should not be
affected significantly.
4.7.1. Speed control block
The main difference is that the model implements only simplified Speed reference
block, which means that the model provides neither four acceleration and deceleration ramps nor
S-curve ramp. Although I have designed and fully tested flip-flop circuit with delays, which
implements four acceleration and deceleration ramps, I could not use this block in the model
since I used sample time of 1μs and I was warned by Simulink that delay blocks may behave in
an unpredictable way, which , in fact, really occurred. The solution to this problem could be to
use different sample times for individuals block. This, however, could lead to problems with
transition rates and further testing would be required. As a result, Speed control block
implements only one acceleration and one deceleration ramp and speed is limited to the minimal
and maximal value. Since the model is always in close-loop, there was no need to implement
Open loop start-up block and also Speed feedback is always determined from tachometer. The
model could be set not in six modes, but only in four modes, since speed torque negative and
speed torque positive mode were omitted. Last difference is that the model does not include CAP
55
IMPLEMENTATION OF POWERFLEX7000 IN SIMULINK
Current Calculator; the reason is that I did not figure out how to calculate the current supplied by
the capacitor. The calculation is not trivial since the circuit depends on the frequency and also the
current must be split into two orthogonal parts.
4.7.2. Flux control block
The flux controller does not include CAP Current Calculator. The reason is the
same as in Speed control block. Next difference is that Flux command limit block does not take
into account Rated Motor voltage, since I have not figured out the correlation between Rated
motor voltage, Rated line voltage and Vline bridge. The last difference is that the model uses
close-loop calculation of Isq command.
4.7.3. Current control block
Current control block, which computes Alpha-line, works exactly according to the
given specification. The only minor difference is that Vdc feedforward is obtained directly by
dividing the magnitude of the stator voltage by V line Average. Whereas PowerFlex7000
measures the voltage in one-phase and then using Alpha Machine (phase) to calculate the
magnitude of the stator voltage. This alternation was done because there is a problem with the
phase-shift. In order to tune the model more effectively, I add a block which only limits the
maximal machine angle. This adjustment has proven to improve the control of DC link in same
cases. In terms of vector modulator, I must admit that this block is implemented probably
differently than in PoweFlex7000. However, there is no exact documentation of such a modulator
and, moreover, this block may be implemented by hardware (e.g. phase-locked loop).
4.7.4. Motor model block
This block includes two main sub-blocks: Voltage flux model and Current flux
model. Voltage flux model is to certain degree know-how of Rockwell Automation since it is
designed by hardware. The reason is simple. Vector control hugely depends on motor parameters
and, therefore, PowerFlex7000 somehow tries to reduce the impact of uncertainties of motor
56
CONCLUSION
parameters by combining software and hardware. On the other hand, Current control block is
implemented by software and the algorithm is well described in [2] or [3].
5. Conclusion
This thesis has helped me to understand different types of control of not only induction
machines but also DC machines. Although this work was practically oriented, I have acquainted
myself with the concept of a vector control. Since induction machines are the most common
actuators and due to the accessibility of microcontrollers and power electronic devices, the vector
control will attract more attention in the future. Hence, I could deal with induction machines and
with Vector control in the future even more intensively.
Nonetheless, the main aim which was to simulate PowerFlex7000 with Siemens ARNRY-
6 has not been accomplished. The reason may be that I have not managed to find out the flux
profile in this motor.
In this section I would like to summarize the objectives which have been accomplished
but also the problems which have not been solved or have been solved only partially and will
need to be addressed in the future.
5.1. Accomplishments
In this thesis I achieved following objectives:
1) I have acquainted myself with the concept of vector control.
2) I have demonstrated the concepts, disadvantages and advantages of vector control
in simulations of 3HP and 200HP motor in both speed and torque regulations.
3) I have implemented an algorithm of Vector control in Simulink.
5.2. Unsolved problems
In this thesis I have not solved following problems:
1) I have not managed to tune the frequency converter for motor Siemens ARNRY-6.
2) I have not solved the problem with the phase shift
3) I have not fully implemented the model of PowerFlex7000.
57
LITERATURE
6. Literature
[1] Rockwell Automation: PowerFlex 7000, Medium Voltage AC Drive “B” Frame (Air-
Cooled), User Manual (Publication 7000-UM150F-EN-P), Cambridge, Canada 2005
[2] Bimal K. Bose (editor), Adjustable Speed AC Drive Systems, Institute of Electrical
and Electronics Engineers, New York 1981
[3] Karel Zeman, Zdeněk Peroutka, Martin Janda: Automatická regulace pohonů s
asynchronními motory, Západočeská univerzita, Elektrotechnická fakulta, Plzeň 2004
[4] Gilbert Sybille: SimPoweSystems 4, User’s Guide, Natick, MA 2007 [5] Jan Bašta: Teorie elektrických strojů, Nakladatelství technické literatury Alfa, Praha
1968
[6] Arias Pujol: Improvements in direct torque control of induction motors, Doctoral
thesis, Technical University of Catalonia 2001
58
LITERATURE
59
APPENDIXES
7. Appendixes
In this section I will not only show results from simulations but also demonstrate typical
properties of frequency converters. This section is divided into following parts:
1) Induction machine 3HP, speed regulation below the base speed, both directions,
pump type load, Vector control
2) Demonstration of the torque ripple and the phase shift
3) Induction machine 3HP, speed regulation above the base speed, one direction,
pump type load, comparison between the flux-constant region and the power-
constant region
4) Induction machine 3HP, torque regulation
5) Induction machine 200HP, speed regulation below the base speed, both directions,
pump type load, reaction to disturbances
6) Induction machine 200HP, speed regulation above the base speed, one direction,
pump type load
7) Induction machine 200HP, heavy duty start-up, traction type load, comparison
between the start-up of induction machines with scalar frequency converters,
vector frequency converters and the start-up of DC motors
8) Induction machine 200HP, torque regulation
9) Induction machine 200HP, torque regulation, uncertainty in all parameters
10) Induction machine 200HP, torque regulation, uncertainty in one parameter
11) Robustness of Vector control
12) Comparison between 3HP, 200HP induction machine and PowerFlex7000
I implemented a model of the pump type load in order to define speed-torque characteristic of the motor. Pump has its torque proportional to the square of the speed:
2ωKTl = This type of the load was used in all speed simulations with the exception of simulation 7. The scheme of all simulations is shown in Figure 25.
60
APPENDIXES
Figure 25 – Schema of the simulation
7.1. Induction machine 3HP: speed regulation below the base speed, both directions, pump type load, Vector control
There are three set points (-800,800 and 1400 rpm). The type of the load is a pump. I will
use the results of this simulation to demonstrate the torque ripple and also the phase shift in the
following section.
7.1.1. Parameters
Parameter Length [s] 10 Solver ode45(Dormant-Prince) Relative tolerance 1e-3 Ts
2[s] 1e-6 Table 24 – Parameters of the simulation
2 Model was discretized by GUI SimPowerToolbox
61
APPENDIXES
Parameter Type pump K [N.m/rpm2] 6e-6
Table 25 – Parameters of the load
Parameter Mechanical input Torque Rotor type Squirrel-cage Reference frame Rotor Nominal power [kW] 2.238 Voltage line-line [V] 220 Base speed [rpm] 1760 Nominal torque [N.m]3
12.14 Stator resistance [ohm] 0.435 Stator leakage inductance [H] 2e-3 Rotor resistance [ohm] 0.816 Rotor leakage inductance [H] 2e-3 Magnetizing(mutual) inductance [H] 69.31e-3 Inertia [kg.m2] 0.2 Friction factor [N.m.s] 0.005 Pole pairs 2
Table 26 – Parameters of the motor
Parameter Speed controller Regulation type Speed Speed controller-proportional gain 2 Speed controller-integral gain 12 Speed measurement - low-pass filter cut-off frequency [Hz] 50
Controller output torque saturation [N.m] [negative, positive] [-15,15]
Torque rate limit [N.m/s] [negative, positive] [-10000,10000]
Maximal speed [rpm] 2500 Minimal speed [rpm] 50 Speed ramp [rmp/s] [negative, positive] [-1500,1500] Flux controller Speed feedback – low-pass filter cut-off frequency [Hz] 100
Flux controller - flux estimation low-pass filter cut-off frequency [Hz] 16
Flux controller - proportional gain 30 Flux controller - integral gain 40
3 Nominal torque was calculated:
nn
PM
ω= where P [W] is the nominal power and nω is the base speed [rad/s]
62
APPENDIXES
Flux controller - flux limit [Wb] [negative, positive] [-20,20]
Nominal torque [Nm] 15 Base speed [rpm] 1760 Initial flux [Wb] 0.3 Nominal flux [Wb] 0.3 Rated line voltage [V] 300 Current controller Current controller - proportional gain 1 Current controller - integral gain 1 Current controller - phase limit [rad] [negative, positive] [-1,1]
Relative current limit [negative, positive] [-1000,1000]
Current rate limit [negative, positive] [-10000,10000] Current feedback – low-pass filter cutoff frequency [Hz] 100
Voltage feedback – low-pass filter cutoff frequency [Hz] 1
Phase feedback – low-pass filter cutoff frequency [Hz] 8
Phase controller – proportional gain 100 Phase controller - integral gain 100 Phase controller - phase limit [rad] [negative, positive] [-10,10]
Phase limit [deg] [lower, upper] [0,120] Switching frequency of vector modulator [kHz] 50
Motor model Initial machine flux (Wb) 0.3 Line side converter gating and feedback
Synchronizing frequency of rectifier [Hz] 60
Pulse width of rectifier [Deg] 10 Snubber resistance of thyristors [ohm] 500 Snubber capacitance of thyristors [F] 1e-6 On-state resistance of thyristors [ohm] 1e-9 Line side converter gating and feedback
Snubber resistance [ohm] 10 Snubber capacitance [F] 1e-6 On-state resistance [ohm] 1e-9 AC Source Amplitude 300 Frequency 60
Table 27 – Parameters of the frequency converter4 4 Parameters regarding induction machine were set exactly the same as induction machine
63
APPENDIXES
7.1.2. Results
Figure 26 – Speed characteristics
Figure 27 – Torque characteristics
64
APPENDIXES
Figure 28 – Flux characteristics
Figure 29 – Current characteristics
65
APPENDIXES
Figure 30 – Id,Iq current
Figure 31 – Id,Iq command
66
APPENDIXES
Figure 32 – Current in detail and the phase shift
Figure 33 – Torque in detail
67
APPENDIXES
Set point [rpm] -800 800 1400 Tr [s]5
1.052 1.992 1.528 Ts [s]6 1.114 2.13 1.702 OS [%] 0.1 0.1 0.04 Steady-state error [rpm] 0.7 0.6 0.2 Steady-state error [%]7
0.04 0.034 0.01 Table 28 – Results of the simulation
7.1.3. Vector control
This simulation shows very important aspect of Vector control, which is the synchronism
between the rotor flux and the stator current. This is shown in Figure 32, where the stator current
is synchronized with the current reference which is determined by the position of the rotor flux. If
the frequency and the amplitude of the stator current was the same as the current reference but the
phase was different, the motor could not be controlled. This simulation has also shown that the
torque is defined by Iq current, since the torque always follows this current. Whereas the flux is
determined by Id current and since the flux is almost constant this current is almost constant as
well. Figure 29 shows two modes of operation. First one is magnetizing, which lasts some 80ms
and during this mode there is only Id component of the stator current, Iq component is zero and,
therefore, the torque is zero as well. When the field reaches a certain level of saturation there is a
normal mode of operation which is characterized by non-zero value of Iq and non-zero value of Id
current resulting in non-zero value of the torque. It could be also observed in Figure 31 that Iq and
Id current interacts with each other. Although Iq command is not correlated with Id command or
with the flux (Iq command depends on the flux reference which is constant in this simulation),
each fluctuation of Id current and, therefore, of Id command is accompanied by the fluctuation of
Iq current and, therefore, of Iq command and visa versa. This shows that these currents interact
within the motor itself. Moreover, this simulation has proven that Iq current could be changed
immediately resulting in minimal overshot of the speed.
Figure 33 shows torque ripple which will be discussed in the following section.
5 Time when the speed reaches 90% of the reference 6 Time when the speed settles between 5% of the steady-state value 7 Related to the base speed
68
APPENDIXES
7.2. Demonstration of the torque ripple and the phase shift
7.2.1. Torque ripple
1) Theoretical background
In an ideal three-phase induction machine, which is fed by a balanced
harmonic current, the flux cannot be produced by the third harmonic and,
therefore, only harmonics of ( )thn 16 ± order must be taken into account.
Furthermore, the ( harmonic produces the flux rotating in the opposite
direction to that produced by the fundamental frequency, whereas the
)thn 16 −
( )thn 16 +
harmonic produces the flux rotating in the same direction to that produced by the
fundamental frequency. The torque defined by (27) could be also defined as a
reaction between the rotor flux and the stator one (in fact, definition (27) is based
on the same principle). So the torque could be also defined as follows:
srrsrse kfT υψψψψ sin)( == (35)
, where srυ is the angle between the stator and the rotor flux and k is the machine
constant.
From (35) follows that the angle is the same if the frequency of the stator and the
rotor flux (current) is the same. If the angle is the same then the torque is always
steady. Since the angle varies at the rate which is the difference between the
speeds of fluxes (currents) then the torque pulsation is at frequencies . n6
2) Analysis of the input signal
In order to show that the pulsation of the torque at 6th harmonic is caused by the
stator current of 5th and 7th harmonic, I inspected the frequency spectrum of the stator
current. Moreover, in order to demonstrate that the modulation I used reduces the torque
pulsation significantly, I compared two other modulation used in CSI (current-source
inverters). Since there is the problem with the phase shift I did not use the current
obtained from the simulation but I used signal, which well approximates the stator current
(Signal 3):
69
APPENDIXES
Figure 34 – Common modulations used in CSI
With the help of Signal processing toolbox, I obtained following FFT spectrum:
Figure 35 – FFT spectrum of stator currents
70
APPENDIXES
Figure 36 – FFT spectrum in detail
Signal 1 2 3 1st/5th 4.91 5.02 23.56 1st/7th 6.87 6.99 51.2
Table 29 – Harmonic distortion of the input signal caused by 5th and 7th
So it is obvious that the modulation I used is far better than the others as far as the
suppressing of the ripple is concerned. The previous simulation (see Figure 33) shows that the
torque ripple is at the 6th harmonic of the fundamental frequency. There is not need to use FFT
analyses or autocorrelation function in order to prove this since it could be easily calculate the
number of ripples in each period since there is the problem with the phase shift.
71
APPENDIXES
7.2.2. Phase shift
Since the inverter and also the induction machine or rather the equivalent circuit
diagram depend on the frequency, it was necessary to implement a phase regulator. In order to
demonstrate the phase shift, I used following simulation:
Figure 37 – Schema of the simulation demonstrating the phase shift
The advantage of this simulation scheme over the frequency converter with the induction
machine is that I could use wider range of frequencies and also the frequency could be changed
immediately. The disadvantage is that only the stator part of the induction machine was modeled.
However, this cannot affect the demonstration of the principle and the solution of the phase shift.
I obtained following results:
72
APPENDIXES
Figure 38 – Demonstration of the phase shift
The text implies the frequency in Hz which was used. The discontinuities in the phase are
caused by a significant phase shift. This simulation shows two different phase-shifts:
a) Off-set phase shift
The reason of this shift is that the vector modulator and the inverter
are not set properly. However, this problem could be easily fixed by
adding off-set phase to the reference phase.
b) Frequency phase shift
This shift is caused by capacitors of the inverter and inductors of
the induction machine. In fact, the induction machine depends even
more on the frequency since the rotor (which was not modeled in this
simulation) depends on the slip (see Figure 9). This problem could be
fixed only by employing a regulator. The regulator compensates not
only the off-set shift but mainly the frequency-induced shift. The
problem I had to face up was the response of the regulator. Since
when the phase changes it is necessary to react as fast as possible in
order not to step out of the synchronism between the stator current
and the rotor flux. In order to achieve this it is important, on one
73
APPENDIXES
hand, to keep the cut-off frequency of the phase low-pass filter as
high as possible. On the other hand, the phase is modulated and,
therefore, the phase must be filtered. So the solution I have come up
with is to modulate the current at very high frequency (tens of kHz).
In fact, this makes the current very similar to the signal 3 in Figure 32
and then the cut-off frequency of phase low-pass filter could be
higher (tens of Hz). This solution or rather compromise should be
solved in order to model PowerFlex7000 more closely. The most
effective solution would be to somehow make the phase continuous,
which would result in reducing the modulation frequency.
The phase shift affects the performance of the system very negatively. It worsens not only
the accuracy but also the dynamics of the system. The phase shift is noticeable in all
characteristics (speed, torque, current, flux).
7.3. Induction machine 3HP: speed regulation above the base speed, one direction, pump type load, comparison between the flux-constant region and the power-constant region
There are three set points (1700, 2000 and 2200 rpm). The type of the load is a pump. I
will use results of this simulation to compare the flux-constant region and the power-constant
region.
7.3.1. Parameters
Parameter Length [s] 8 Solver ode45(Dormant-Prince) Relative tolerance 1e-3 Ts
8[s] 1e-6 Table 30 – Parameters of the simulation
Parameter
Type pump K [N.m/rpm2] 2e-6
8 Model was discretized by GUI SimPowerToolbox
74
APPENDIXES
Table 31 – Parameters of the load
Parameter Mechanical input Torque Rotor type Squirrel-cage Reference frame Rotor Nominal power [kW] 2.238 Voltage line-line [V] 220 Base speed [rpm] 1760 Nominal torque [N.m] 12.14 Stator resistance [ohm] 0.435 Stator leakage inductance [H] 2e-3 Rotor resistance [ohm] 0.816 Rotor leakage inductance [H] 2e-3 Magnetizing(mutual) inductance [H] 69.31e-3 Inertia [kg.m2] 0.2 Friction factor [N.m.s] 0.005 Pole pairs 2
Table 32 – Parameters of the motor
Parameter Speed controller Regulation type Speed Speed controller-proportional gain 2 Speed controller-integral gain 12 Speed measurement - low-pass filter cut-off frequency [Hz] 50
Controller output torque saturation [N.m] [negative, positive] [-15,15]
Torque rate limit [N.m/s] [negative, positive] [-10000,10000]
Maximal speed [rpm] 2500 Minimal speed [rpm] 50 Speed ramp [rmp/s] [negative, positive] [-1500,1500]
Flux controller Speed feedback – low-pass filter cut-off frequency [Hz] 100
Flux controller - flux estimation low-pass filter cut-off frequency [Hz]
16
Flux controller - proportional gain 30 Flux controller - integral gain 40 Flux controller - flux limit [Wb] [negative, positive] [-20,20]
Nominal torque [Nm] 15 Base speed [rpm] 1760 Initial flux [Wb] 0.3
75
APPENDIXES
Nominal flux [Wb] 0.3 Rated line voltage [V] 300 Current controller Current controller - proportional gain 1
Current controller - integral gain 1 Current controller - phase limit [rad] [negative, positive] [-1,1]
Relative current limit [negative, positive] [-1000,1000]
Current rate limit [negative, positive] [-10000,10000]
Current feedback – low-pass filter cutoff frequency [Hz] 100
Voltage feedback – low-pass filter cutoff frequency [Hz] 1
Phase feedback – low-pass filter cutoff frequency [Hz] 8
Phase controller - proportional gain 100 Phase controller - integral gain 100 Phase controller - phase limit [rad] [negative, positive] [-10,10]
Phase limit [deg] [lower, upper] [0,120] Switching frequency of vector modulator [kHz] 50
Motor model Initial machine flux (Wb) 0.3 Line side converter gating and feedback
Synchronizing frequency of rectifier [Hz] 60
Pulse width of rectifier [Deg] 10 Snubber resistance of thyristors [ohm] 500
Snubber capacitance of thyristors [F] 1e-6 On-state resistance of thyristors [ohm] 1e-9
Line side converter gating and feedback
Snubber resistance [ohm] 10 Snubber capacitance [F] 1e-6 On-state resistance [ohm] 1e-9 AC Source Amplitude 300 Frequency 60
Table 33 – Parameters of the frequency converter9
9 Parameters regarding induction machine were set exactly the same as induction machine
76
APPENDIXES
7.3.2. Results
Figure 39 – Speed characteristics
Figure 40 – Torque characteristics
77
APPENDIXES
Figure 41 – Flux characteristics
Figure 42 – Current characteristics
78
APPENDIXES
Figure 43 – Id,Iq current
Figure 44 – Id, Iq command
79
APPENDIXES
Set point [rpm] 1700 2000 2200 Tr [s] 2.374 0.69 0.636 Ts [s] 2.548 0.73 0.69 OS [%] 0.18 0.11 0.07 Steady-state error [rpm] 2 0.6 0.32 Steady-state error [%] 0.11 0.034 0.018
Table 34 – Results of the simulation
7.3.3. Comparison between the flux-constant region and the power-constant region
The frequency converters operate in two different modes- flux-constant region (below the
base speed) and power-constant region (under the base speed) which are shown in Figure 45.
However, PowerFlex7000 allows the flux to be set below the base speed not constant but
proportional to the torque, but in majority cases the initial flux is set equal to the nominal flux (as
it happens in each simulation). Above the base speed the flux is reduced resulting in lower torque
and, therefore, the power is constant (speed has increased). From the previous simulation could
be observed that the drive delivers nominal torque above the base speed which results in
overloading the motor. As far as PowerFlex7000 is concerned the control of the torque is more
complex, since there is a factory setting which defines different power modes of the operation
(heavy-duty and normal-duty), which means that the motor could be overloaded but only for
certain time. This is done by limiting the output current.
However, the result from the previous simulation demonstrates that when the flux
decreases (see Figure 41) the torque also decreases and if the drive should deliver the nominal
torque above the base speed it is necessary to overload the motor. This could be observed in
Figure 44 which shows that in order to deliver the nominal torque above the base speed the Iq
command has to be increased compared to Iq command below the base speed.
Figure 45 – Operation modes of frequency converters
80
APPENDIXES
7.4. Induction machine 3HP, torque regulation
There are 10 torque set points (5, 0, 1, 4, 8, 14,-5, 11, -2 and 8 N.m). This simulation
should show an open loop simulation, since the speed PI regulator is to be by-passed. In order to
reduce the speed there is a constant load of 4 N.m.
7.4.1. Parameters
Parameter Length [s] 10 Solver ode45(Dormant-Prince) Relative tolerance 1e-3 Ts
10[s] 1e-6 Table 35 – Parameters of the simulation
Parameter
Mechanical input Torque Rotor type Squirrel-cage Reference frame Rotor Nominal power [kW] 2.238 Voltage line-line [V] 220 Stator resistance [ohm] 0.435 Stator leakage inductance [H] 2e-3 Rotor resistance [ohm] 0.816 Rotor leakage inductance [H] 2e-3 Magnetizing(mutual) inductance [H] 69.31e-3 Inertia [kg.m2] 0.2 Friction factor [N.m.s] 0.005 Pole pairs 2
Table 36 – Parameters of the motor
Parameter Speed controller Regulation type Torque Controller output torque saturation [N.m] [negative, positive] [-15,15]
Torque rate limit [N.m/s] [negative, positive] [-10000,10000] Flux controller Speed feedback – low-pass filter cut-off frequency [Hz] 100
Flux controller – flux estimation low-pass filter cut-off frequency [Hz] 16
Flux controller – proportional gain 30
10 Model was discretized by GUI SimPowerToolbox
81
APPENDIXES
Flux controller – integral gain 40 Flux controller – flux limit [Wb] [negative, positive] [-20,20]
Nominal torque [Nm] 15 Base speed [rpm] 1760 Initial flux [Wb] 0.3 Nominal flux [Wb] 0.3 Rated line voltage [V] 300 Current controller Current controller – proportional gain 1 Current controller – integral gain 1 Current controller – phase limit [rad] [negative, positive] [-1,1]
Relative current limit [negative, positive] [-1000,1000] Current rate limit [negative, positive] [-10000,10000] Current feedback – low-pass filter cutoff frequency [Hz] 100
Voltage feedback – low-pass filter cutoff frequency [Hz] 1
Phase feedback – low-pass filter cutoff frequency [Hz] 8
Phase controller – proportional gain 100 Phase controller – integral gain 100 Phase controller – phase limit [rad] [negative, positive] [-10,10]
Phase limit [deg] [lower, upper] [0,120] Switching frequency of vector modulator [kHz] 50
Motor model Initial machine flux (Wb) 0.3 Line side converter gating and feedback Synchronizing frequency of rectifier [Hz] 60 Pulse width of rectifier [Deg] 10 Snubber resistance of thyristors [ohm] 500 Snubber capacitance of thyristors [F] 1e-6 On-state resistance of thyristors [ohm] 1e-9 Line side converter gating and feedback Snubber resistance [ohm] 10 Snubber capacitance [F] 1e-6 On-state resistance [ohm] 1e-9 AC Source Amplitude 300 Frequency 60
Table 37 – Parameters of the frequency converter11
11 Parameters regarding induction machine were set exactly the same as induction machine
82
APPENDIXES
7.4.2. Results
Figure 46 – Torque characteristics
Figure 47 – Flux characteristics
83
APPENDIXES
Figure 48 – Current characteristics
Figure 49 – Id, Iq current
Since the torque regulation is an open loop one the accuracy is lower than when regulating the speed.
84
APPENDIXES
7.5. Induction machine 200HP: speed regulation below the base speed, both directions, pump type load, reaction to disturbances
There are three speed set points (-1000, 1300 and 900 rpm). The type of the load is again
a pump. Moreover, in order to emulate a disturbance there is a malfunction of the pump at 5.0 s,
which increases immediately the load torque by 400N.m and at 7.5s the torque drops back to the
previous value. This simulation should show not only a speed regulation in both directions, but
also the reaction to disturbances.
7.5.1. Parameters
Parameter Length [s] 20 Solver ode45(Dormant-Prince) Relative tolerance 1e-8 Ts
12[s] 1e-6 Table 38 – Parameters of the simulation
Parameter
Type pump K [N.m/rpm2] 6e-4
Table 39 – Parameters of the load
Parameter Mechanical input Torque Rotor type Squirrel-cage Reference frame Rotor Nominal power [kW] 150 Voltage line-line [V] 460 Base speed [rpm] 1785 Nominal torque [N.m] 800 Frequency [Hz] 60 Stator resistance [ohm] 0.01818 Stator leakage inductance [H] 0.00019 Rotor resistance [ohm] 0.009956 Rotor leakage inductance [H] 0.00019 Magnetizing(mutual) inductance [H] 0.009415 Inertia [kg.m2] 8 Friction factor [N.m.s] 0.04789 Pole pairs 2
12 Model was discretized by GUI SimPowerToolbox
85
APPENDIXES
Table 40 – Parameters of the motor
Parameter Speed controller Regulation type Speed Speed controller-proportional gain 5 Speed controller-integral gain 20 Speed measurement - low-pass filter cut-off frequency [Hz] 50
Controller output torque saturation [N.m] [negative, positive] [-1200,1200]
Torque rate limit [N.m/s] [negative, positive] [-5000,5000]
Maximal speed [rpm] 2500 Minimal speed [rpm] 50 Speed ramp [rmp/s] [negative, positive] [-900,900] Flux controller Speed feedback – low-pass filter cut-off frequency [Hz] 100
Flux controller - flux estimation low-pass filter cut-off frequency [Hz] 16
Flux controller - proportional gain 100 Flux controller - integral gain 30 Flux controller - flux limit [Wb] [negative, positive] [-2,2]
Nominal torque [Nm] 1200 Base speed [rpm] 1785 Initial flux [Wb] 0.73 Nominal flux [Wb] 0.73 Rated line voltage [V] 500 Current controller Current controller - proportional gain 1 Current controller - integral gain 1 Current controller - phase limit [rad] [negative, positive] [-1,1]
Relative current limit [negative, positive] [-100,100]
Current rate limit [negative, positive] [-1000,1000] Current feedback – low-pass filter cutoff frequency [Hz] 100
Voltage feedback – low-pass filter cutoff frequency [Hz] 1
Phase feedback – low-pass filter cutoff frequency [Hz] 10
Phase controller - proportional gain 100 Phase controller - integral gain 100 Phase controller - phase limit [rad] [negative, positive] [-10,10]
Alpha line [deg] [lower, upper] [0.01,180]
86
APPENDIXES
Switching frequency of vector modulator [kHz] 50
Motor model Initial machine flux (Wb) 0.73 Line side converter gating and feedback
Synchronizing frequency of rectifier [Hz] 60
Pulse width of rectifier [Deg] 10 Snubber resistance of thyristors [ohm] 500 Snubber capacitance of thyristors [F] 1e-6 On-state resistance of thyristors [ohm] 1e-9 Line side converter gating and feedback
Snubber resistance [ohm] 1 Snubber capacitance [F] 1e-5 On-state resistance [ohm] 1e-9 AC Source Amplitude 500 Frequency 60 DC link Inductance [H] 0.25 Resistance [ohm] 0.5
Table 41 – Parameters of the frequency converter13
13 Parameters regarding induction machine were set exactly the same as the induction machine
87
APPENDIXES
7.5.2. Results
Figure 50 – Speed characteristics
Figure 51 – Torque characteristics
88
APPENDIXES
Figure 52 – Flux characteristics
Figure 53 – Current characteristics
89
APPENDIXES
Figure 54 – Id,Iq characteristics
Set point [rpm] -1000 1300 900 Tr [s] 1.043 2.41 0.48 Ts [s] 1.793 2.99 0.52 OS [%] 4 1 1.7 Steady-state error [rpm] 1.6 1 1.7 Steady-state error [%] 0.1 0.06 0.1
Table 42 – Results of the simulation
7.5.3. Reaction to disturbances
It could be observed that even the disturbances were significant (50% of the nominal
torque); the frequency converter coped well and was able to recover the steady state speed within
1000ms. Furthermore, the overshot was minimal.
90
APPENDIXES
7.6. Induction machine 200HP: speed regulation above the base speed, one direction, pump type load
There are three speed set points (1700, 2000 and 2200 rpm). The type of the load is again
a pump. This simulation should show how the model behaves above the base speed, which means
how the flux changes, torque changes and, consequently, how the speed is affected by these
changes.
7.6.1. Parameters
Parameter Length [s] 13 Solver ode45(Dormant-Prince) Relative tolerance 1e-8 Ts
14[s] 1e-6 Table 43 – Parameters of the simulation
Parameter
Type pump K [N.m/rpm2] 16e-5
Table 44 – Parameters of the load
Parameter Mechanical input Torque Rotor type Squirrel-cage Reference frame Rotor Nominal power [kW] 150 Voltage line-line [V] 460 Base speed [rpm] 1785 Nominal torque [N.m] 800 Frequency [Hz] 60 Stator resistance [ohm] 0.01818 Stator leakage inductance [H] 0.00019 Rotor resistance [ohm] 0.009956 Rotor leakage inductance [H] 0.00019 Magnetizing(mutual) inductance [H] 0.009415 Inertia [kg.m2] 8 Friction factor [N.m.s] 0.04789 Pole pairs 2
14 Model was discretized by GUI SimPowerToolbox
91
APPENDIXES
Table 45 – Parameters of the motor
Parameter Speed controller Regulation type Speed Speed controller-proportional gain 5 Speed controller-integral gain 20 Speed measurement – low-pass filter cut-off frequency [Hz] 50
Controller output torque saturation [N.m] [negative, positive] [-1200,1200]
Torque rate limit [N.m/s] [negative, positive] [-5000,5000]
Maximal speed [rpm] 2500 Minimal speed [rpm] 50 Speed ramp [rmp/s] [negative, positive] [-900,900]
Flux controller Speed feedback – low-pass filter cut-off frequency [Hz] 100
Flux controller - flux estimation low-pass filter cut-off frequency [Hz]
16
Flux controller - proportional gain 100 Flux controller - integral gain 30 Flux controller - flux limit [Wb] [negative, positive] [-2,2]
Nominal torque [Nm] 1200 Base speed [rpm] 1785 Initial flux [Wb] 0.73 Nominal flux [Wb] 0.73 Rated line voltage [V] 500 Current controller Current controller - proportional gain 1
Current controller - integral gain 1 Current controller – phase limit [rad] [negative, positive] [-1,1]
Relative current limit [negative, positive] [-100,100]
Current rate limit [negative, positive] [-1000,1000]
Current feedback – low-pass filter cutoff frequency [Hz] 100
Voltage feedback – low-pass filter cutoff frequency [Hz] 1
Phase feedback – low-pass filter cutoff frequency [Hz] 10
Phase controller - proportional gain 100 Phase controller – integral gain 100
92
APPENDIXES
Phase controller - phase limit [rad] [negative, positive] [-10,10]
Alpha line [deg] [lower, upper] [0.01,180] Switching frequency of vector modulator [kHz] 50
Motor model Initial machine flux (Wb) 0.73 Line side converter gating and feedback
Synchronizing frequency of rectifier [Hz] 60
Pulse width of rectifier [deg] 10 Snubber resistance of thyristors [ohm] 500
Snubber capacitance of thyristors [F] 1e-6 On-state resistance of thyristors [ohm] 1e-9
Line side converter gating and feedback
Snubber resistance [ohm] 1 Snubber capacitance [F] 1e-5 On-state resistance [ohm] 1e-9 AC Source Amplitude 500 Frequency 60 DC link Inductance [H] 0.25 Resistance [ohm] 0.5
Table 46 – Parameters of the frequency converter15
15 Parameters regarding induction machine were set exactly the same as the induction machine
93
APPENDIXES
7.6.2. Results
Figure 55 – Speed characteristics
Figure 56 – Torque characteristics
94
APPENDIXES
Figure 57 – Flux characteristics
Figure 58 – Current characteristics
95
APPENDIXES
Figure 59 – Id,Iq characteristics
Figure 60 – Current at full speed
Set point [rpm] 1700 2000 2200 Tr [s] 1.69 0.79 - Ts [s] 1.83 1.146 - OS [%] 3 17 - Steady-state error [rpm] 2 2 - Steady-state error [%] 0.11 0.12 -
Table 47 – Results of the simulation
96
APPENDIXES
It could be observed that above the speed of 2000rpm, the frequency converter is not able to follow the reference. The reason is that DC link is not capable of delivering the reference current (see Figure 58). Figure 60 shows the stator current at full speed. It is obvious (compare with Figure 32) that at higher speeds the stator current is more sinusoidal since as the speed
increases the ability of the stator windings of smoothing the stator current improves ( 1
/−fRL ss is
higher, f is the frequency of the drive).
7.7. Induction machine 200HP: heavy duty start-up, traction type load, comparison of the start-up of induction machines with scalar frequency converters, vector frequency converters and the start-up of DC motors
This simulation should demonstrate that the vector frequency converters are capable of
delivering breakaway torque of 150% of the nominal one.
In this simulation I will use traction type of the load (e.g. locomotive). The typical feature
of this load is very high breakaway torque, but almost constant running torque. This simulation
models only simplified model of the locomotive; it does not take into account the fact that since
the motor is connected to the locomotive by the gear-box it produces the force which makes the
train accelerate. At the same time the speed of the train determines the speed of the motor and
due to the high weight of the train the acceleration would last much longer than in the simulation.
The traction type of the load was modeled as follows:
400780+=
vTl [N.m], where the speed of the motor was limited to: 100,1∈v [rpm]
97
APPENDIXES
7.7.1. Parameters
Parameter Length [s] 3 Solver ode45(Dormant-Prince) Relative tolerance 1e-8 Ts
16[s] 1e-6 Table 48 – Parameters of the simulation
Parameter
Mechanical input Torque Rotor type Squirrel-cage Reference frame Rotor Nominal power [kW] 150 Voltage line-line [V] 460 Base speed [rpm] 1785 Nominal torque [N.m] 800 Frequency [Hz] 60 Stator resistance [ohm] 0.01818 Stator leakage inductance [H] 0.00019 Rotor resistance [ohm] 0.009956 Rotor leakage inductance [H] 0.00019 Magnetizing(mutual) inductance [H] 0.009415 Inertia [kg.m2] 40 Friction factor [N.m.s] 0.04789 Pole pairs 2
Table 49 – Parameters of the motor
Parameter Speed controller Regulation type Speed Speed controller-proportional gain 20 Speed controller-integral gain 20 Speed measurement – low-pass filter cut-off frequency [Hz] 50
Controller output torque saturation [N.m] [negative, positive] [-1200,1200]
Torque rate limit [N.m/s] [negative, positive] [-50000,50000] Maximal speed [rpm] 2500 Minimal speed [rpm] 50 Speed ramp [rmp/s] [negative, positive] [-10000,10000] Flux controller Speed feedback – low-pass filter cut-off frequency [Hz] 100
16 Model was discretized by GUI SimPowerToolbox
98
APPENDIXES
Flux controller - flux estimation low-pass filter cut-off frequency [Hz] 16
Flux controller - proportional gain 100 Flux controller - integral gain 30 Flux controller - flux limit [Wb] [negative, positive] [-2,2] Nominal torque [Nm] 1200 Base speed [rpm] 1785 Initial flux [Wb] 0.73 Nominal flux [Wb] 0.73 Rated line voltage [V] 500 Current controller Current controller - proportional gain 1 Current controller - integral gain 1 Current controller – phase limit [rad] [negative, positive] [-1,1]
Relative current limit [negative, positive] [-100,100] Current rate limit [negative, positive] [-1000,1000] Current feedback – low-pass filter cutoff frequency [Hz] 100 Voltage feedback – low-pass filter cutoff frequency [Hz] 1
Phase feedback – low-pass filter cutoff frequency [Hz] 10 Phase controller - proportional gain 100 Phase controller – integral gain 100 Phase controller - phase limit [rad] [negative, positive] [-10,10] Alpha line [deg] [lower, upper] [0.01,180] Switching frequency of vector modulator [kHz] 50 Motor model Initial machine flux (Wb) 0.73 Line side converter gating and feedback Synchronizing frequency of rectifier [Hz] 60 Pulse width of rectifier [deg] 10 Snubber resistance of thyristors [ohm] 500 Snubber capacitance of thyristors [F] 1e-6 On-state resistance of thyristors [ohm] 1e-9 Line side converter gating and feedback Snubber resistance [ohm] 1 Snubber capacitance [F] 1e-5 On-state resistance [ohm] 1e-9 AC Source Amplitude 500 Frequency 60 DC link Inductance [H] 0.25 Resistance [ohm] 0.5
Table 50 – Parameters of the frequency converter17
17 Parameters regarding induction machine were set exactly the same as the induction machine
99
APPENDIXES
7.7.2. Results
Figure 61 – Speed characteristics
Figure 62 – Torque characteristics
100
APPENDIXES
Figure 63 – Torque v Speed
Figure 64 – Current characteristics
101
APPENDIXES
7.7.3. Comparison of the start-up of induction machines with scalar frequency converters, vector frequency converters and the start-up of DC motors
This simulation (see Figure 62) has shown that the induction machine controlled by the vector frequency converter is able to deliver 150 % breakaway torque. This feature is significant advantage over scalar frequency converters, which speed-torque characteristics is shown in Figure 65. Vector control enables induction machines to be used in traction and, hence, nowadays trend is to replace DC series excitation motors (high breakaway torque) by induction machines. The best examples of this trend are suburban train 471 and pendolino.
Figure 65 – Speed/Torque characteristic of scalar frequency converters
7.8. Induction machine 200HP: torque regulation
Let us imagine this situation. We are supposed to control the power of a coil power plant.
There is a coil conveyor, which load varies in an unpredictable way. So the amount of coil and,
therefore, the power could be controlled by changing the torque of the coil conveyor (P=kM).
102
APPENDIXES
7.8.1. Parameters
Parameter Length [s] 10 Solver ode45(Dormant-Prince) Relative tolerance 1e-8 Ts
18[s] 1e-6 Table 51 – Parameters of the simulation
Parameter
Mechanical input Torque Rotor type Squirrel-cage Reference frame Rotor Nominal power [kW] 150 Voltage line-line [V] 460 Base speed [rpm] 1785 Frequency [Hz] 60 Stator resistance [ohm] 0.01818 Stator leakage inductance [H] 0.00019 Rotor resistance [ohm] 0.009956 Rotor leakage inductance [H] 0.00019 Magnetizing(mutual) inductance [H] 0.009415 Inertia [kg.m2] 8 Friction factor [N.m.s] 0.04789 Pole pairs 2
Table 52 – Parameters of the motor
Parameter Speed controller Regulation type Torque Controller output torque saturation [N.m] [negative, positive] [-1200,1200]
Torque rate limit [N.m/s] [negative, positive] [-5000,5000] Flux controller Speed feedback – low-pass filter cut-off frequency [Hz] 100
Flux controller - flux estimation low-pass filter cut-off frequency [Hz] 16
Flux controller - proportional gain 100 Flux controller - integral gain 30 Flux controller - flux limit [Wb] [negative, positive] [-2,2] Nominal torque [Nm] 1200 Base speed [rpm] 1785 Initial flux [Wb] 0.73
18 Model was discretized by GUI SimPowerToolbox
103
APPENDIXES
Nominal flux [Wb] 0.73 Rated line voltage [V] 500 Current controller Current controller - proportional gain 1 Current controller - integral gain 1 Current controller - phase limit [rad] [negative, positive] [-1,1]
Relative current limit [negative, positive] [-100,100] Current rate limit [negative, positive] [-1000,1000] Current feedback – low-pass filter cutoff frequency [Hz] 100
Voltage feedback – low-pass filter cutoff frequency [Hz] 1
Phase feedback – low-pass filter cutoff frequency [Hz] 10 Phase controller - proportional gain 100 Phase controller - integral gain 100 Phase controller - phase limit [rad] [negative, positive] [-10,10] Alpha line [deg] [lower, upper] [0.01,180] Switching frequency of vector modulator [kHz] 50 Motor model Initial machine flux (Wb) 0.73 Line side converter gating and feedback Synchronizing frequency of rectifier [Hz] 60 Pulse width of rectifier [Deg] 10 Snubber resistance of thyristors [ohm] 500 Snubber capacitance of thyristors [F] 1e-6 On-state resistance of thyristors [ohm] 1e-9 Line side converter gating and feedback Snubber resistance [ohm] 1 Snubber capacitance [F] 1e-5 On-state resistance [ohm] 1e-9 AC Source Amplitude 500 Frequency 60 DC link Inductance [H] 0.25 Resistance [ohm] 0.5
Table 53 – Parameters of the frequency converter19
19 Parameters regarding induction machine were set exactly the same as the induction machine
104
APPENDIXES
7.8.2. Results
Figure 66 – Torque characteristics
Figure 67 – Flux characteristics
105
APPENDIXES
Figure 68 – Current characteristics
Figure 69 – Id,Iq current
106
APPENDIXES
7.9. Induction machine 200HP: torque regulation, uncertainty in all parameters
This simulation and the following one should point out the disadvantage of vector
converters – sensitiveness to uncertainties in motor parameters. Since during the operation the
parameters of the motor could change, I will perform two simulations. In the first simulation I
will set motor parameters used in the frequency converter to 150% of the nominal motor
parameters. The others parameters of the simulation are exactly the same as in simulation 5.8.
Since it is more likely that only some parameters change (e.g. resistance varies with
temperature), in second simulation I will set only motor resistances used in the frequency
converter to 150% of the nominal motor resistance. The others parameters of the simulation are
exactly the same as in simulation 5.8.
7.9.1. Results
Figure 70 – Torque characteristics
107
APPENDIXES
Figure 71 – Flux characteristics
Figure 72 – Id,Iq current
7.10. Induction machine 200HP: torque regulation, uncertainty in one parameter
7.10.1. Results
108
APPENDIXES
Figure 73 – Torque characteristics
Figure 74 – Flux characteristics
109
APPENDIXES
Figure 75 – Id,Iq current
7.11. Robustness of Vector control
Simulations 5.9 and 5.10 have shown one significant drawback of Vector control –
sensitiveness to uncertainties in parameters of the motor. In order to compare results from such
simulations, it is important to realize that Vector control is based on equation (31), (32), (33) and
(34):
( ) ( )sis
Ls sdr
mr τ
ψ+
=1
msqrr
mr pi
LLR ω
ψωψ +=
1
msd L
i ψ=
ψM
LL
pi
m
rsq
132
=
It is obvious that (34) is not affected by the change in all parameters, as long as the
change is proportional. Therefore, Iq command is the same (Figure 72). Equation (32) (position of
the flux) is also not affected by the proportional change in all parameters, since the magnitude of
110
APPENDIXES
the flux rψ changed according to (31). So the only equations which are affected by the change in
all parameters are (31) and (33). Hence, the flux shown in Figure 71 has been calculated wrongly
and, therefore, the flux profile in the motor is not 0.73Wb but lower resulting in field- weakening
mode (see section 5.3) and, therefore, electromagnetic torque of the motor is also lower (Figure
70).
If only one parameter is changed, all equations are affected and the motor cannot be
controlled (see Figure 73).
7.12. Comparison between 3HP, 200HP induction machine and PowerFlex7000
I will compare performance of the frequency converter with 3HP motor and 220HP motor
at the speed set point of 1700rpm.
7.12.1. Comparison between 3HP and 200HP induction machine
Model 3HP 200HP Tr [s] 2.374 1.69 Ts [s] 2.548 1.83 OS [%] 0.18 3 Steady-state error [rpm] 2 2 Steady-state error [%] 0.11 0.11
JTn [N.kg-1m-1]
75 150
Range of current [A] 15 300
1700l
n
TT
2.6 2.6
Table 54 – Comparison between 3HP and 200HP at 1700rpm
The reason why 200HP motor accelerates quicker than 3HP is that3
3
200
200
JT
JT nn > . Since it is more
difficult to control wider range of current 200HP motor has bigger overshot. However, both motors maintained the same steady-state error.
111
APPENDIXES
7.12.2. Comparison between the model and PowerFlex7000
According to [1] Rockwell Automation guarantees following speed regulations:
Frequency [Hz] Below 6 Above 6 PowerFlex7000 [%]20 0.02 0.01 Equivalent speed [rpm] Below 360 Above 900 Model-3HP [%] - 0.11 Model-200HP [%] - 0.11
Table 55 – Comparison between the model and PowerFlex7000 at 1700rpm
As far as a steady-state error is concerned I have not managed to obtain similar accuracy
as PowerFlex7000. This was probably caused by the phase shift.
20 Error related to the base speed
112